According to standard Combine doctrine, a corvette is a rapid response ship under 100m, whose purpose in fleet actions is to quickly reinforce formations as they fall under duress from hostile counterattack. The role of a frigate, a ship between 100m and 150m, on the other hand, is to provide long range support for fleet engagements, and though lightly armored for their class, are nimble enough to easily relocate to suit the needs of the fleet.
We will be posting more on Combine fleet docrtine at a later date.