Generally, a user is a person who has a Surfly account. Agent and user are interchangeable terms, as most Surfly users are agents.
A customer who starts a co-browsing session from your website is not a user, but the agent who takes the call is a user.
Surfly’s license is user-based, so a seat refers to a license for one user. Any seat can create an unlimited number of sessions, but each seat is to be used by a single user only.