The main difference is the voids in the inner plys as described. The "A" and "C" refer to the grades of the veneer used in the layup. "A" sheets originally were free of knots and splits. Many years ago the grading rules were changed to allow patches and still have an "A" grade. The patches are either the little football shaped inserts or can also be a poly patch applied after the sheet is pressed. The "C" grade sheets are used in inner plys and also for the back sheet. "A-C" sheets have an "A" grade face but the core and back sheets are "C" grade or better which allow knots and splits.

For true marine use you don't want any voids in the plywood so all "A" sheets are used. That is quite expen$ive. For bulkheads and other things not exposed to a lot of moisture the A-C sheets are structurally sound. One thing to be careful of are the sheets that are a fir or pine panel with a mahogany or other hardwood face sheet. The are nice and smooth and a good choice for painting but unless they are graded "exterior" they probably do not use exterior type resin for the layup. For interior work I like using sheets of Apple Ply that are made up of thin veneers of hardwood. The ones I get here in Portland, Oregon are with alder cores and decorative faces. Lovely to work with and not much more expensive than the fir panels with the mahogany faces. BEWARE: there are also some nice looking panels at Home Depot with thin core veneers but those are made with cottonwood (also referred to as "Poplar") but are complete junk. The ones I tried out a while back came from China. You are better off with chip board.