Non defining relative clauses are used to give extra information about someone or something. If we didn't say them, the information would still seem complete:
Peter is a lawyer + (he is my best friend) > Peter, who is my best friend, is a lawyer.
The river Nile flows towards the North. + (it is the longest river in the world) > The river Nile, which is the longest river in the world, flows towards the North.
These clauses do NOT define the person or object BECAUSE IT IS ALREADY DEFINED, so usually the refernet of the relative pronoun is one of these:
The information in non-defining clauses is unncesessary to understand the main clause, and so it is always separated from it with commas or comma and full stop.
Moreover, the pronoun THAT can never be used in this type of sentence, and it is not possible to omit the relative pronouns.