Most window systems use a desktop metaphor. Part of this metaphor is the idea that system-level windows (representing, e.g., Emacs frames) are stacked in a notional third dimension perpendicular to the screen surface. The order induced by stacking is total and usually referred to as stacking (or Z-) order. Where the areas of two windows overlap, the one higher up in that order will (partially) cover the one underneath.
You can raise a frame to the top of that order or lower a frame to its bottom by using the functions raise-frame and lower-frame. You can restack a frame directly above or below another frame using the function frame-restack.
Note that all functions described below will respect the adherence of frames (and all other window-system windows) to their respective z-group (see Position Parameters). For example, you usually cannot lower a frame below that of the desktop window and you cannot raise a frame whose z-group parameter is nil above the window-system’s taskbar or tooltip window.
This function raises frame frame (default, the selected frame) above all other frames belonging to the same or a lower z-group as frame. If frame is invisible or iconified, this makes it visible. If frame is a child frame (see Child Frames), this raises frame above all other child frames of its parent.
This function lowers frame frame (default, the selected frame) below all other frames belonging to the same or a higher z-group as frame. If frame is a child frame (see Child Frames), this lowers frame below all other child frames of its parent.
This function restacks frame1 below frame2. This implies that if both frames are visible and their display areas overlap, frame2 will (partially) obscure frame1. If the optional third argument above is non-nil, this function restacks frame1 above frame2. This means that if both frames are visible and their display areas overlap, frame1 will (partially) obscure frame2.
Technically, this function may be thought of as an atomic action performed in two steps: The first step removes frame1’s window-system window from the display. The second step reinserts frame1’s window into the display below (above if above is true) that of frame2. Hence the position of frame2 in its display’s Z (stacking) order relative to all other frames excluding frame1 remains unaltered.
Some window managers may refuse to restack windows.
Note that the effect of restacking will only hold as long as neither of the involved frames is iconified or made invisible. You can use the z-group (see Position Parameters) frame parameter to add a frame to a group of frames permanently shown above or below other frames. As long as a frame belongs to one of these groups, restacking it will only affect its relative stacking position within that group. The effect of restacking frames belonging to different z-groups is undefined. You can list frames in their current stacking order with the function frame-list-z-order (see Finding All Frames).
If this is non-nil, activation of the minibuffer raises the frame that the minibuffer window is in.
On window systems, you can also enable auto-raising (on frame selection) or auto-lowering (on frame deselection) using frame parameters. See Management Parameters.
The concept of raising and lowering frames also applies to text terminal frames. On each text terminal, only the top frame is displayed at any one time.
This function returns the top frame on terminal. terminal should be a terminal object, a frame (meaning that frame’s terminal), or nil (meaning the selected frame’s terminal). If it does not refer to a text terminal, the return value is nil.
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https://www.gnu.org/software/emacs/manual/html_node/elisp/Raising-and-Lowering.html