Ancillary Power Pools
Ancillary Power Pools (also known as Epic Power Pools) are powersets that become available for Heroes at level 41. Your choices are limited by your Archetype. Warshades and Peacebringers do not get Ancillary Power Pools.
Generally, the powers in these pools are selected from other Archetypes, and are designed to give each Archetype access to types of abilities that they do not normally have. Holds and ranged attacks for melee Archetypes, armors for ranged Archetypes, etc.
Be very careful when choosing an Ancillary Power Pool. Once selected, you cannot switch power pools, EVEN WITH A RESPEC. If attempted, the respec will only offer the single APP group that you have chosen.
For Villains, the equivalent choices (Level 41+, unable to be altered once chosen) are called Patron Power Pools.
Blaster
Blaster Ancillary Powersets give them slight Scrapper-like resistances, small Tanker-like self boosts, and small crowd control capability.
Controller
Controller Ancillary Powersets give them slight Scrapper-like resistances, and some (ranged) offensive capability.
Defender
Defender Ancillary Powersets give them slight Scrapper-like resistance and some self-boosting powers; Psychic Mastery gives them slight control capability.
Scrapper
Scrapper Ancillary Pools give them one or two ranged powers, and slight crowd control capability.
Tanker
Tanker Ancillary Powersets give them slight crowd control capability, and some ranged offense.