For situations where you want to add/remove items from a group, entry is arbitrary, but ordering is fixed.
For example:
- adding user to a rights group; you could add users that don't exist in the DB yet, so you can select user names which are banned from logging in or being created; once selected, they're always shown alphabetically, or
- tagging a media file; you could add new tags that you're imagining as well as selecting from existing ones, but once selected they're always shown in popularity order.
