support levels for both offense and defense show exactly how much either one can help the other do in terms of attacking or shielding an ally from damage. Ex. Within the Tower you can unlock Gin Ichimaru if memory serves he starts with atk support A Lvl. what this means is that when he supports whoever if u pay close attention when he attacks he does a good bit of damage. S is the highest in either attk support or damage repelling in def support. Just watch Gin With S atk and compare it to anyone with C,B, and A u will see a huge difference in damage but depending upon how u place the points on ur characters u may not see it as much with def supports. Hope this helps