An action commission is a subset of regular commissions. The only difference is that action commissions do not generate upline commissions. It is a one-off commission amount you can generate for specific actions.
An example of an action commission would be if a user fills out registration form for you. This would generate a commission in your affiliate program. However, if you are using multiple commission levels, and the referring user has a sponsor / up-line, no commissions would get generated for them, only the direct referring affiliate.