Non-identifying relative clause
A non-identifying relative clause, also known as a non-defining or non-restrictive relative clause is a relative clause that adds secondary information to a sentence, almost as an afterthought. Commas are used to "isolate" the clause from the rest of the sentence.
- My brother, who is an engineer, has been working in the same company for over twenty-five years.
- My favourite dish is roast beef and Yorkshire pudding, which is difficult to find on a restaurant menu.
 You cannot use that
- John, who (not that) plays rugby and cricket, is always busy at weekends.
- Paul took me for a drive in his new car, which (not that) he had bought last week.
 You cannot leave out who or which
- We stayed at the Grand Hotel, which Ann had recommended to us.
This is one of the few cases that you can use whom (when it is the object):
- This morning I met Diane, who (or whom) I hadn’t seen since Christmas.