Global Auto Revert Mode normally only reverts file buffers. There are
two ways to auto-revert certain non-file buffers: by enabling Auto
Revert Mode in those buffers (using M-x auto-revert-mode); and
by setting global-auto-revert-non-file-buffers to a
non-nil value. The latter enables Auto Reverting for all types
of buffers for which it is implemented (listed in the menu below).
Like file buffers, non-file buffers should normally not revert while you are working on them, or while they contain information that might get lost after reverting. Therefore, they do not revert if they are modified. This can get tricky, because deciding when a non-file buffer should be marked modified is usually more difficult than for file buffers.
Another tricky detail is that, for efficiency reasons, Auto Revert often does not try to detect all possible changes in the buffer, only changes that are major or easy to detect. Hence, enabling auto-reverting for a non-file buffer does not always guarantee that all information in the buffer is up-to-date, and does not necessarily make manual reverts useless.
At the other extreme, certain buffers automatically revert every
auto-revert-interval seconds. (This currently only applies to
the Buffer Menu.) In this case, Auto Revert does not print any
messages while reverting, even when auto-revert-verbose is
non-nil.
Some non-file buffers can be updated reliably by file notification on
their default directory; Dired buffers is an example. The major mode
can indicate this by setting buffer-auto-revert-by-notification
to a non-nil value in that buffer, allowing Auto Revert to
avoid periodic polling. Such notification does not include changes to
files in that directory, only to the directory itself.
The details depend on the particular types of buffers and are explained in the corresponding sections.