Ok, but Susan Daughtrey Punctuation Book 1, pages 26 & 27 gives this example:
The boy who rides the yellow bicycle is my friend.
Do you need to know the boy who rides the yellow bike to understand who the sentence is talking about?
Does this sentence require commas?
No, there should not be any commas in this sentence. Without who rides a yellow bike
we would not know who the friend is. This information is essential to the meaning of the sentence. Commas are not necessary.
Then, in her exercises, she gives these questions:
Exercise 15, page 28:
The china plate which already have a crack in it broke when I dropped it.
In her answer scheme, no commas are necessary. You need the information about the crack in the plate for the meaning of the sentence to be complete.
On the other hand:
The village store which sells everything from crisps to crockery is run by Mrs Bailey. In the answer scheme, commas are necessary. This is because the information "which sells everything from crisps to crockery"
is not absolutely necessary for the meaning of the sentence to be complete.