Pronominalization is the use of a pronoun instead of a noun. Relativization is when a noun or noun phrase in an embedded sentence is deleted or pronominalized (see Complementizers post).
Pronominalization is appropriate when the noun in an embedded clause (a constituent part of another clause) is identical to the noun in the matrix clause (which contains the embedded clause). In referring to the noun in the matrix clause, a resumptive pronoun (a co-referential noun phrase) is often used. Look at these examples where the noun in the matrix clauses are replaced by the pronoun it in the embedded clauses:
SUMMARY/OBSERVATIONS: When a co-referential noun phrase appears inside another relative clause, it is not deleted but instead pronominalized. Notice also that each of the resumptive pronouns is obligatory because the verb preceding it is transitive.