Active Directory UID Mapping to Specific Unix UIDs

I really don't understand the translation of Active Directory UIDs Mapping to Unix UIDs. When a person logs in with their AD credentials how does winbind understand that it needs to map that AD UID to a specific Unix UID, which is tied to a home directory and their personal files. All I see in config files is that winbind specifies a range of UIDs that can be used, and that's about it.