A non restrictive adjective clause dose not
define its subject,but only gives more
information about it.

The president of Pakistan who had just returned from
Saudi-Arabia was received at the airport by high officials

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[A]In this sentences the subject is well-defined and the
adjective clause who had just returned from Saudi Arabia
only gives some additional information about the president
of Pakistan.It is a non-restrictive or non-defining adjective
clause.

[B]A non-restrictive adjective clause is separated from its
subject by a comma.A comma also marks its end.