On items, I think it would be appropriate to edit the "Equipped appearance" as a string which does not currently appear to be an option. Also, YW should probably be "Wings, Male" and ZW should be "Wings, Female". Both originate from 1pp: Attachable Wings.It is possible, but only indirectly. Right-click on the field and select "Edit as hex". Right-click again and select (the now available) "Edit as string". I'll see if I can expand the datatype for "Equipped appearance" to allow setting custom strings.
I don't know what YW stands for. In BGEE it doesn't seem to work. The only item with YW appearance is WING01B.ITM, which isn't used by the game. ZW doesn't look gender-specific. It can also be used by both male and female characters.
Read the dev_readme.txt in the 1pp: Attachable Wings archive. It explains the difference between the two and the reason for it. Regardless of who and where else uses any of these resources, this mod by Erephine is the origin of the YW and ZW equipped appearances. As the EEs include much of 1pp, it does not surprise me that bits of this got slipped in with the rest.