[Koha] Missing Genres (and confusion over 942$c and 952$y)

[Chris Brown]

Gentle Reader,

We are using a minimalist Marc framework for adding books. We really only
need Author, Title, and Genre. We have defined  a set of itemtypes
corresponding to our genres.

Our problem is that values (selected from a dropdown) in the 952$y field on
our "Add MARC record" screen do not reach the items.itype field in the
database. In fact as far as we can tell they aren't being stored ANYWHERE,
even though the "Koha to MARC Mapping" screen shows that 952$y is mapped to
items.itype.

See screenshot here:

https://www.dropbox.com/s/5bs9ipwh9y48ajv/genre-entry.PNG?dl=0

I would be grateful if anyone has an explanation for this or suggestions on
how to fix it.

More generally, can anyone point me at an explanation of how the 942$c and
952$y fields are related? They appear to have the same purpose in life.

Thanks and Best Regards,