For our premium systems such as Corporate Edition, all licensing is based on concurrent usage. For example, if you buy access for 10 staff, then you'll be able to get any 10 staff running the software simultaneously. That could be from a potential pool of, say, 20 staff who are logging in and out of the software as needed.
Or in other words, let's say you had 7 staff who were likely to be in the software all day and a half dozen users who might need to get in there occasionally. You'd probably be safe, then, with a 10 user licence.
For other editions such as Professional or Enterprise, licencing is per PC. However, the software allows you to change the mix of PC's that are licensed on your network once a session ends. The end of a session is defined as when all users log out of your database.