A package is either a simple package or a multi-file package. A simple package is stored in a package archive as a single Emacs Lisp file, while a multi-file package is stored as a tar file (containing multiple Lisp files, and possibly non-Lisp files such as a manual).
In ordinary usage, the difference between simple packages and multi-file packages is relatively unimportant; the Package Menu interface makes no distinction between them. However, the procedure for creating them differs, as explained in the following sections.
Each package (whether simple or multi-file) has certain attributes:
A short word (e.g., ‘auctex’). This is usually also the symbol prefix used in the program (see Coding Conventions).
A version number, in a form that the function version-to-list understands (e.g., ‘11.86’). Each release of a package should be accompanied by an increase in the version number so that it will be recognized as an upgrade by users querying the package archive.
This is shown when the package is listed in the Package Menu. It should occupy a single line, ideally in 36 characters or less.
This is shown in the buffer created by C-h P (describe-package), following the package’s brief description and installation status. It normally spans multiple lines, and should fully describe the package’s capabilities and how to begin using it once it is installed.
A list of other packages (possibly including minimal acceptable version numbers) on which this package depends. The list may be empty, meaning this package has no dependencies. Otherwise, installing this package also automatically installs its dependencies, recursively; if any dependency cannot be found, the package cannot be installed.
Installing a package, either via the command package-install-file, or via the Package Menu, creates a subdirectory of package-user-dir named name-version, where name is the package’s name and version its version (e.g., ~/.emacs.d/elpa/auctex-11.86/). We call this the package’s content directory. It is where Emacs puts the package’s contents (the single Lisp file for a simple package, or the files extracted from a multi-file package).
Emacs then searches every Lisp file in the content directory for autoload magic comments (see Autoload). These autoload definitions are saved to a file named name-autoloads.el in the content directory. They are typically used to autoload the principal user commands defined in the package, but they can also perform other tasks, such as adding an element to auto-mode-alist (see Auto Major Mode). Note that a package typically does not autoload every function and variable defined within it—only the handful of commands typically called to begin using the package. Emacs then byte-compiles every Lisp file in the package.
After installation, the installed package is loaded: Emacs adds the package’s content directory to load-path, and evaluates the autoload definitions in name-autoloads.el.
Whenever Emacs starts up, it automatically calls the function package-activate-all to make installed packages available to the current session. This is done after loading the early init file, but before loading the regular init file (see Startup Summary). Packages are not automatically made available if the user option package-enable-at-startup is set to nil in the early init file.
This function makes the packages available to the current session. The user option package-load-list specifies which packages to make available; by default, all installed packages are made available. See Package Installation in The GNU Emacs Manual.
In most cases, you should not need to call package-activate-all, as this is done automatically during startup. Simply make sure to put any code that should run before package-activate-all in the early init file, and any code that should run after it in the primary init file (see Init File in The GNU Emacs Manual).
This function initializes Emacs’ internal record of which packages are installed, and then calls package-activate-all.
The optional argument no-activate, if non-nil, causes Emacs to update its record of installed packages without actually making them available.
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https://www.gnu.org/software/emacs/manual/html_node/elisp/Packaging-Basics.html