It gets the key for the file. From my experience each file in the DB has a unique key what I believe is a number of the order that items are entered into the DB (DB item number). If an item is removed it just leaves a hole. So Key = last item entered key +1.
If you have the key to a file you can get the IMJFileAutomation for the file using IMJAutomation.GetFileByKey which I would guess is faster than IMJAutomation.GetFile(Filename as String) since it does not require a search to find the item in the DB. Or add items to playlist using IMJCurPlaylistAutomation.AddFileByKey instead of IMJCurPlaylistAutomation.AddFile. I ran a test and adding ~12000 items using AddFile took ~21 sec. vs 9 sec. using AddFileByKey.
BTW these have been around for a while and only recently add to the SDK docs.