One 'relative' difference:
In defining relative clauses, that can be, and often is, used instead of which:
The book that (which) I am reading at the moment is very interesting.
In non-defining relative clauses, only which can be used:
This book, which is about colour, was written by a blind man.
Which, but not that can refer to a situation as well as a noun.:
John decided not to come, which was a great relief.
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