Non-restrictive Clause

A non-restrictive clause is a clause in which the noun phrase that is used to avoid repetition (as the referent of an anaphor, meaning that it is substituted by another word, but refers to the same noun) is determined by its antecedent where the dependent is peripheral (non-essential) in the secondary constituent as opposed to a restrictive clause where the dependent is central (essential) to its primary constituent. A nonrestrictive clause does not identify the referent of its noun, but only provides information about it.

Restrictive example:

The officer helped the civilians who had been shot.

or

The officer helped those civilians who had been shot.

In this example, there is no comma before "who." Therefore, what follows is a restrictive clause. (Not all of the civilians had been shot.)

Non-restrictive example:

The officer helped the civilians, who had been shot.

Here is a comma before "who." Therefore, what follows is a non-restrictive clause. It changes the sentence to mean that all the civilians had been shot.