A relativizer is the relative pronoun that, who/whom or which, and each is used to form a subject or object relative clause.
In the following examples, the rule of ordinary relative clause formation is such that the relativizer (bolded) is identical to the noun in the matrix noun phrase (italicized). Also the head-initialized antecedent (bolded) is marked by that, which is the relativizer and the syntactic head of a full clause:
SUMMARY/OBSERVATIONS: As a result of this above-mentioned rule acting upon the relative clause, it, which has been pronominalized (using a pronoun instead of a noun), is to be deleted because it is unnecessary. This is a standard approach to relativization, whereby a noun or noun phrase in an embedded sentence is deleted or pronominalized.