ACCTivate! uses a concurrent license model to manage the number of users that can access the software. Here’s an excerpt from a wikipedia article that explains the benefits:
Concurrent user licensing allows firms to purchase computer systems and software at a lower cost because the maximum number of concurrent users expected to use the system or software at any given time (those users all logged in together) is only a portion of the total system users employed at a company. The concurrent licenses are global and shared by anyone who needs to use the system. This contrasts with “named-seats” licensing in which one license must be purchased for each and every individual user whether they are using the system or not.
This model allows ACCTivate! to be installed on many computers within your organization for convenient access, but restricts the number of users that can use the software at the same time.