A determiner has the type-value rel if the NP of which the determiner is a constituent has an antecedent in the main clause and if the NP realizes a verb argument of the clause in which it occurs.
Given NP the nominal phrase, X a determiner, N a nou, P the main clause, P' the clause in which NP occurs, If NP(X+N) realizes a position of a verb argument of P' and if NP(X+N) has an antecedent in P, X is a relative determiner.