There are actually a number of license models and not all apply to all versions. These include:
concurrent user - total number of users on a given server active at one time. V9 and upgrade to OE10.
registered client - total number of connected devices, whether active or not and regardless of how many different people use that device.
named user - total number of people, whether currently active or not, regardless of how many devices they use.
access agent - based on number of agents required to support the user load, but limited to the use of occasional or unknown user types. Occasional is logged in, but use less than 2 hours a week and unknown is not logged in and accessing from a public web site.
All but concurrent user are OE10. There are lots of little details about batch processes etc. Note that a "user" does not need to be directly connected to a Progress database to count as a user if they are using the data in a Progress database.