Restrictive relative clause
GENERAL: Relative clause which is used to restrict the class of entities that can be denoted by a noun phrase.
EXAMPLE: in the books that John read, the restrictive relative
clause that John read restricts the set of books to those that are read
by John. Non-restrictive relative clauses add further qualifications to the
reference of the noun phrase but do not narrow down (nor expand) its extension.
Thus in this book, which John gave to me, the non-restrictive relative
clause does not restrict the set of books. The difference between a restrictive
and a non-restrictive interpretation is often only expressed intonationally.