An action function is a function display-buffer calls for choosing a window to display a buffer. Action functions take two arguments: buffer, the buffer to display, and alist, an action alist (see Buffer Display Action Alists). They are supposed to return a window displaying buffer if they succeed and nil if they fail.
The following basic action functions are defined in Emacs.
This function tries to display buffer in the selected window. It fails if the selected window is a minibuffer window or is dedicated to another buffer (see Dedicated Windows). It also fails if alist has a non-nil inhibit-same-window entry.
This function tries to display buffer by finding a window that is already displaying it. Windows on the selected frame are preferred to windows on other frames.
If alist has a non-nil inhibit-same-window entry, the selected window is not eligible for reuse. The set of frames to search for a window already displaying buffer can be specified with the help of the reusable-frames action alist entry. If alist contains no reusable-frames entry, this function searches just the selected frame.
If this function chooses a window on another frame, it makes that frame visible and, unless alist contains an inhibit-switch-frame entry, raises that frame if necessary.
This function tries to display buffer by finding a window that is displaying a buffer in a given mode.
If alist contains a mode entry, its value specifes a major mode (a symbol) or a list of major modes. If alist contains no mode entry, the current major mode of buffer is used instead. A window is a candidate if it displays a buffer whose mode derives from one of the modes specified thusly.
The behavior is also controlled by alist entries for inhibit-same-window, reusable-frames and inhibit-switch-frame, like display-buffer-reuse-window does.
This function tries to display buffer by splitting the largest or least recently-used window (usually located on the selected frame). It actually performs the split by calling the function specified by split-window-preferred-function (see Choosing Window Options).
The size of the new window can be adjusted by supplying window-height and window-width entries in alist. If alist contains a preserve-size entry, Emacs will also try to preserve the size of the new window during future resize operations (see Preserving Window Sizes).
This function fails if no window can be split. More often than not, this happens because no window is large enough to allow splitting. Setting split-height-threshold or split-width-threshold to lower values may help in this regard. Splitting also fails when the selected frame has an unsplittable frame parameter; see Buffer Parameters.
This function tries to display buffer in a window where it was displayed previously.
If alist contains a non-nil inhibit-same-window entry, the selected window is not eligible for use. A dedicated window is usable only if it already shows buffer. If alist contains a previous-window entry, the window specified by that entry is usable even if it never showed buffer before.
If alist contains a reusable-frames entry (see Buffer Display Action Alists), its value determines which frames to search for a suitable window. If alist contains no reusable-frames entry, this function searches just the selected frame if display-buffer-reuse-frames and pop-up-frames are both nil; it searches all frames on the current terminal if either of those variables is non-nil.
If more than one window qualifies as usable according to these rules, this function makes a choice in the following order of preference:
previous-window alist entry, provided it is not the selected window. previous-window alist entry or showed buffer before. This function tries to display buffer by choosing an existing window and displaying the buffer in that window. It can fail if all windows are dedicated to other buffers (see Dedicated Windows).
This function tries to display buffer at a location specified by alist. For this purpose, alist should contain a direction entry whose value is one of left, above (or up), right and below (or down). Other values are usually interpreted as below.
If alist also contains a window entry, its value specifies a reference window. That value can be a special symbol like main which stands for the selected frame’s main window (see Side Window Options and Functions) or root standing for the selected frame’s root window (see Windows and Frames). It can also specify an arbitrary valid window. Any other value (or omitting the window entry entirely) means to use the selected window as reference window.
This function first tries to reuse a window in the specified direction that already shows buffer. If no such window exists, it tries to split the reference window in order to produce a new window in the specified direction. If this fails as well, it will try to display buffer in an existing window in the specified direction. In either case, the window chosen will appear on the side of the reference window specified by the direction entry, sharing at least one edge with the reference window.
If the reference window is live, the edge the chosen window will share with it is always the opposite of the one specified by the direction entry. For example, if the value of the direction entry is left, the chosen window’s right edge coordinate (see Coordinates and Windows) will equal the reference window’s left edge coordinate.
If the reference window is internal, a reused window must share with it the edge specified by the direction entry. Hence if, for example, the reference window is the frame’s root window and the value of the direction entry is left, a reused window must be on the left of the frame. This means that the left edge coordinate of the chosen window and that of the reference window are the same.
A new window, however, will be created by splitting the reference window such that the chosen window will share the opposite edge with the reference window. In our example, a new root window would be created with a new live window and the reference window as its children. The chosen window’s right edge coordinate would then equal the left edge coordinate of the reference window. Its left edge coordinate would equal the left edge coordinate of the frame’s new root window.
Four special values for direction entries allow to implicitly specify the selected frame’s main window as the reference window: leftmost, top, rightmost and bottom. This means that instead of, for example, (direction . left) (window . main) one can just specify (direction . leftmost). An existing window alist entry is ignored in such cases.
This function tries to display buffer in a window below the selected window. If there is a window below the selected one and that window already displays buffer, it reuses that window.
If there is no such window, this function tries to create a new window by splitting the selected one, and displays buffer there. It will also try to adjust that window’s size provided alist contains a suitable window-height or window-width entry, see above.
If splitting the selected window fails and there is a non-dedicated window below the selected one showing some other buffer, this function tries to use that window for showing buffer.
If alist contains a window-min-height entry, this function ensures that the window used is or can become at least as high as specified by that entry’s value. Note that this is only a guarantee. In order to actually resize the window used, alist must also provide an appropriate window-height entry.
This function tries to display buffer in a window at the bottom of the selected frame.
This either tries to split the window at the bottom of the frame or the frame’s root window, or to reuse an existing window at the bottom of the selected frame.
This function creates a new frame, and displays the buffer in that frame’s window. It actually performs the frame creation by calling the function specified in pop-up-frame-function (see Choosing Window Options). If alist contains a pop-up-frame-parameters entry, the associated value is added to the newly created frame’s parameters.
This function tries to display buffer in a child frame (see Child Frames) of the selected frame, either reusing an existing child frame or by making a new one. If alist has a non-nil child-frame-parameters entry, the corresponding value is an alist of frame parameters to give the new frame. A parent-frame parameter specifying the selected frame is provided by default. If the child frame should become the child of another frame, a corresponding entry must be added to alist.
The appearance of child frames is largely dependent on the parameters provided via alist. It is advisable to use at least ratios to specify the size (see Size Parameters) and the position (see Position Parameters) of the child frame, and to add a keep-ratio parameter (see Frame Interaction Parameters), in order to make sure that the child frame remains visible. For other parameters that should be considered see Child Frames.
This function tries to display buffer by finding a frame that meets a predicate (by default any frame other than the selected frame).
If this function chooses a window on another frame, it makes that frame visible and, unless alist contains an inhibit-switch-frame entry, raises that frame if necessary.
If alist has a non-nil frame-predicate entry, its value is a function taking one argument (a frame), returning non-nil if the frame is a candidate; this function replaces the default predicate.
If alist has a non-nil inhibit-same-window entry, the selected window is not used; thus if the selected frame has a single window, it is not used.
If alist has a non-nil allow-no-window entry, then this function does not display buffer and returns the symbol fail. This constitutes the only exception to the convention that an action function returns either nil or a window showing buffer. If alist has no such allow-no-window entry, this function returns nil.
If this function returns fail, display-buffer will skip the execution of any further display actions and return nil immediately. If this function returns nil, display-buffer will continue with the next display action, if any.
It is assumed that when a caller of display-buffer specifies a non-nil allow-no-window entry, it is also able to handle a nil return value.
Two other action functions are described in their proper sections—display-buffer-in-side-window (see Displaying Buffers in Side Windows) and display-buffer-in-atom-window (see Atomic Windows).
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https://www.gnu.org/software/emacs/manual/html_node/elisp/Buffer-Display-Action-Functions.html