[Koha] Confusion over "item type"

[Chris Brown]

Hello,

MARC records 942$c and 952$y both appear to specify an "item type". They
appear to be mapped to koha fields biblio.itemtype and items.itype
respectively. Please could someone explain to me the difference between
them? In particular when I define a circulation rule for a specific item
type, which one does it use?

I am using Koha 17.05

Thanks!

Chris Brown (still a newbie, but getting there ...)