Imagine the CALs as bus tickets. The session cal is a ticket that's valid always for the person carrying it and after that person gets off it can be handed to any other person, however, it can only be carried by one person at a time.
The usage CALs are instead a pile of single tickets for the bus. These can be assigned to whoever and grant that person an hour on the bus. If the trip is longer it requires a second ticket. The tickets are renewed on a monthly basis but the renewal is a steady increase and not all at the same time. So if you have 28 usage cals the renewal for each cal is 1/28th of a CAL a day meaning that with 28 CALs one usage CAL per day will be renewed. Seems I drifted away from the bus metaphor but I hope you get the picture.
A Usage CAL is more limited. It's only good for an hour over a 28 day period. This is probably for demo purposes or assuming it is a much lower cost, it may be an effective solution fo clients who intend on spending very little time in the application.
For isntance, if you have ten usage CALs, you can access the QVW for a total of ten hours over 28 days. It could be one 10 hour session or ten 1 hour sessions. But either way, once ten hours is used up, you have to wait until next month to use the server again.