So, with defining relative clauses
We get essential information about a person, object, situation, etc. Without it the sentence wouldn't make any sense.
Characteristics:
1. No commas used on either side of the clause.
2. accompanied by relative pronouns: who, which, whose, where, when, why.
3. who and which can be replaced by that.
4. who, which and that can be omitted when they are the object (lijdend vw).