What Does Seat Mean?
Seat refers to a user granted access to protected digital software. Each person provided with a license granting user-level security is defined as a seat. Licensed software on a server may only be accessed by those who have been granted a seat, also referred to as a seat license. Those with a seat license are identified in the system directory; only they can access the protected software. Thus, each user computer is considered a seat.
Techopedia Explains Seat
Seat licenses can be obtained for each computer depending on how many users there will be. For instance, a seat license for 20 users would permit software usage by 20 different and specifically named users.
Microsoft often uses the per-seat type of license. When there are more computer connections than purchased seat licenses, problems arise. Thus, it is better to use only the seat licenses purchased. Site licenses are more typically used in large companies to allow a larger number of users to access licensed software.