A training unit can be defined as everytime a student initally starts a couse. A training unit could also be described as a single user/course. For example:

5 students each taking 1 course = 5 units
1 student taking 5 courses = 5 units

Extending that out, for your block of 500 units you could have any one of the following scenarios:

500 student taking 1 course each
100 students taking 5 courses each
20 students taking 5 courses each and 100 students taking 2 courses each and 10 students taking 20 courses each.

Pay Per Course is the best option for less than 120 units. If you need more than 120 units, purchasing the 500 block option would be the most cost effective option.