Okay, so then you have to create it first.
In case you haven't done it before:
Edit, Preferences, Tags (1), custom tags, define new tags.
In an empty slot under identifier, type Release Type then press tab and see it fill in the others.
Save, then under custom tags choose a slot and change custom(whatever#) to Release Type and instead of "save to MusicBee database only" choose Release Type and save.
After it scans your files, you should then be able to use the <Album Artist>\<Year (yyyy)>\<Release Type>\<Album>\<Disc-Track#> <Title> format.