I've seen this GUI component for many years, even on the old Apple's.
What's it called???

It looks like this:

               Listbox 1:                    Listbox 2:
               ----------                    ----------

               item1                            item2
               item3            <<            item5
               item4            >>            item9
               item6

                           Ok        Cancel

Crude picture, huh!

Basically it's two lists.
Listbox 1 is populated by selecting items in Listbox 2, then clicking "<<".
Or by selecting item(s) in Listbox 2, then clicking ">>".
When Listbox 1 contains what you want, you hit "Ok" and the operation
executes with whatever is in Listbox 1..

An example is Microsoft Outlook's "To:, Cc:, Bcc:" chooser, that adds names
from an address book to the "To:" field of a mail message.

What's this thing called????

Thanks,
Bill Pieterouiski
Software Engineer
Maranti Networks, Inc.