use of an unspecified value, or other behavior where this International Standard provides two or more possibilities and imposes no further requirements on which is chosen in any instance
This sentence is in C99 which is the official documentation written by ANSI about a programming language. So I think this sentence won't wrong. But
on which is chosen in any instance looks awkward. I have known when a preposition relative pronoun is used, following sentence should be a full(complete) sentence. But a subject is omitted.
How should I interpret this sentence and is it grammatically correct?