Compositions and recordings exist as 2 separate pieces of copyright. Songtrust administers your composition side, while your distributor (Cd Baby, Distrokid, etc.) or label manages the recording side.
Being that not all songs have recordings, we always register all compositions from your Songtrust account regardless of what (if any) recording info is provided.
Here is the difference of including ISRC's, if you do have recordings out there.