The head nouns may explicitly or implicitly also refer to entities situated in the non-argument position of a relative clause. When it refers to a non-argument explicitly, a pronominal element must be placed after a preposition.
For instance, in the relative clauses 我替他畫畫的人 (w˘o t`ı t¯a hu`a hu`a de r´en/ The person, for whom I draw a picture) and 他看書的圖書館 (t¯a z`ai n`a-l˘ı k`an sh¯u de t´ush¯ugu˘an/ The library where he read), the head nouns 人 (r´en/ person) and 圖書館 (t´ush¯ugu˘an/ library) are co-referential to the benefactive 他 (t¯a/ he) and the locative 那裡 (n`a-l˘ı/ there) , respectively.
However, when a relative clause does not contain a prepositional phrase, it is implied that the head noun is co-referential to a non-argument that is empty in the relative clause.
For example, in the relative clause 他看書的圖書館 (w˘o t¯a k`an sh¯u de t´ush¯ugu˘an/ the library where he read), a non-argument such as a locative phrase 在圖書館 (z`ai t´ush¯ugu˘an/ in the library) is implied in the relative clause.
Reference: Chinese: a linguistic introduction by Chao Fen Sun