bug#81512: [PATCH] Add command `outline-occur'

> We should avoid any terms in use like "overriding" or "base" because it
> would confuse users and we must avoid "--" and "internal" because we
> have to document it in the manual.  Since we process it in addition to
> 'display-buffer-alist' we could call it 'display-buffer-add-alist' or
> 'display-buffer-complementary-alist'.

Or 'display-buffer-default-alist' for symmetry with:

  completion-category-defaults  -> display-buffer-default-alist
  completion-category-overrides -> display-buffer-alist
                                   display-buffer-overriding-action

> I'd still like to hear someone's opinion on when and why this should be
> set in _any other way_ but let-binding it.  It would be a package that
> always wants to override ACTION arguments of nested 'display-buffer'
> calls.  As such, it could be useful for the bookmark jump scenario
> described in another thread.  But if a second package wants to do the
> same - and the bookmark discussion seems to imply that there is more
> than one client of a bookmark jump buffer display call - then the
> settings will collide if they use a 'category' entry (and anything but a
> 'category' does not seem very useful for bookmarks).  Please comment on
> that issue before we proceed.

It's unlikely that two packages will use the same category.
Even in this case the user can change the order of package loading
the get the desired behavior for the same category.