Non-defining relative clauses add details, not essential, about the person or thing we are discussing. They are usually set off by commas.
Example
- This music, which I really like, was used at the show.
- My sister, who I truly admire, is coming for Thanksgiving.
In non-defining relative clauses, who and which cannot be replaced with that or omitted.
Example
- This music,
that I really like, was used at the show. ❌- This music, which I really like, was used at the show. ✅
- My sister,
I truly admire, is coming for Thanksgiving. ❌- My sister, whom I truly admire, is coming for Thanksgiving. ✅
References