Dave is one of our salesmen. He spoke to our customer, Susan.
Dave is the salesman who/that spoke to Susan.
Susan is one of our customers. Dave spoke to her.
Susan is the customer Dave spoke to. (no need in English to use who or that)
You can't use that in extra-information clauses:
Dave spoke to Susan, who is our customer.
Dave, who is our salesman, spoke to Susan.
He's the boy. He's new in the class.
Peter invited the boy who/that is new in the class to his party.
Or, as extra information about the boy:
Peter invited the boy, who is new in the class, to his party.