The copula signs are to acknowledge a relationship between two things.
A directional copula has a direct spatial reference while a non-directional copula doesn’t.
There is a sign specifically for a copula that goes before with a WH.
This person is a teacher.
The building is an apartment.
Potatoes are vegetables.
ASL is a language.
Copula (with WH)
This school is where we met.
They are the ones who want to buy the house.