Concurrent users are the number of users logged in at the system ALL AT THE SAME TIME. This is different from total NUMBER OF USERS who may not all be using the system simultaneously at any given time.
A contract with moodLearning to support a system usually contains provision on the number of concurrent users to be supported by the system. A typical moodLearning contract would be sensitive to the number of concurrent users but not necessarily to total number of users.
A learning management system (LMS), for instance, can have thousands of users. But the contract (with moodLearning support) specifies only 500 concurrent users. So what learning managers may do is to schedule use (say, for exam taking) by batches. A one-hour exam, for example, may be rolled out for 2 hours for 1,000 users, each batch of 500 users taking the exam for an hour each. However, in the estimation of learning managers, all the 1,000 users can try to take the exam simultaneously and not over-burden the system. moodLearning just does not guarantee success of such utilization. But it's the learning manager's call.