I only heard it once - radio 4 - philosophy don of course
( quiddity is much commoner)
duns scotus ( yes really ) haeccity is what makes dogs separate
and quiddity what they have in common ( what makes them member of the set of dogs 1950 - or a genus 1450)
1950 - one is a subset of the other ( haeccity is part of quiddity I think)
I havent got down to things nature