A(n) clause should always be separated from the rest of the sentence by commas.

A nonrestrictive clause should always be separated from the rest of the sentence by commas.

Explanation:

A nonrestrictive clause, also called nonessential, is an adjective clause that contains information that is not essential to the sentence: it adds extra details to it but without this clause, the sentence would still make complete sense. These type of clauses should always be separated from the rest of the sentence by commas when placed in the middle of a sentence, or followed by an only comma when they are the last part of the sentence.

Examples (Nonrestrictive clauses underlined):

The Declaration of Independence, which was written by Jefferson, is a document that declares American Independence from the tyrannic rule of Britain.

That is Carlos, who is my sister's fiance.