In what situation would somebody use the clause the car which is yellow?
For example:
If I say the car which is yellow, am I giving you more information about a particular car we were already talking about by telling you its colour, or am I helping you to identify the car by telling you that it is the yellow one I want to discuss, rather than a red or blue one?
We are looking here at two different kinds of relative clause, which we can call non-restrictive and restrictive relative clauses.
The relative clause gives you information to help you identify the correct car and to narrow down our choices from a set of options.
How about this example?
Here, the relative clause adds some extra information about the car, but it was already clear which car we were talking about.
We call this a non-restrictive relative clause.