The user can search by full name, email (when visible), nickname and username, but not by position.
Propose adding support for searching users by position, specified in Account Settings > General > Position in the following lists:
Main Menu > View/Manage Members
Channel drop down menu > Add Members
Channel drop down menu > View/Manage Members
"More..." at the bottom of your direct message list
NOTE: Searching for users by position shouldn't work on the message autocomplete or on the search :from modifier, since it's not visible on the UI.