Disc Set is setting learned E Skills so that they can be used in battle.
At first there will only be six disc slots where E Skills can be set, but Disc Analysis will unlock points and allow for expansion. Each heroine has Mic Skills and E Skills at varying levels of proficiency.
E Skills at a higher proficiency will consume less SP, and ones at a lower proficiency will consume more SP. It should also be noted that E Skills not set will be unusable during battle.