SaaS is licensed different ways. A common approach has been a per-seat license whereby the cost is based on a number of named users. The concurrent licensing method is based on a total number of users online at one time.
The usage-based model is becoming more common whereby the license is based on the total amount of processing power or storage used.
For very complex cloud-computing offerings, per-feature licensing is based on specific capabilities of the software and not the entire application. See
SaaS and
cloud computing.