Clauses: Essential vs. Nonessential
Relative Pronouns: Use Them to Create Essential and Nonessential Dependent Adjective Clauses to Modify a Noun or a Pronoun in the Independent Clause
When referring to people, to pets with names, or to animals with a known gender, use the relative pronouns who, whom, and whose. When referring to places, to things, or to animals without names or a known gender, use whose only.
When referring to places, things, animals without names or a known gender, or groups of people (e.g., panel, jury), use that or which.