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.

Examples
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Directional Copula 
This person is a teacher. 
The building is an apartment.

Non-directional Copula
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. 

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