A generation groups 100 tower floors. In later generations, you can find higher tiers of weapons and armor and face enemies carrying combinations of variants.
Check the target floor and generation in "Tower". Before battle, inspect variants in "Scouting Intel" and review the special actions of the boss at the end of a later generation.
Compare equipment from a new generation with your main party’s gear. Your best generation determines the relic forging limit, so review "Gear" after reaching a new generation.