What is a credit? A credit, or more precisely, the current number of available credits is the determining factor on what licenses you can select. Each license type has an associated number of credits that are required to activate it.
As an example, when you first purchase VCM Suite with 8 credits, it ships with 8 universal credits. You could use these credits to license four LS1 Single vehicles or two LS2 vehicles, or 2 x LS1 and 1 x LS2 etc.
If you do not have enough credits to activate the desired license you must purchase and add more credits. Eg. If you have 4 existing credits and wish to activate an LS1 year/model license (requires 6 credits) you would need to purchase at least 2 additional credits before that option would be available.
Overall, the credit is the basic measure of what licenses you can and can't activate at any time.