The Retention Effects System (보유 효과 시스템, boyu hyogwa sisteutem)ⓘ or Possession Effects are a feature added with the Korean Chapter 5 patch in 2017, which unlocks party effects for the player's account when specific Transcended or Overlord Units are owned.
The effects are comparable to the old Party Effects, where Units in a party can trigger unique effects when paired with one another. However, that restriction is not present with this system, allowing more flexible parties. Therefore, while Party Effects required the specific units to activate the effect, the Retention Effects unlock the effect just by owning the required Transcended/Overlords.
The system was added as Flint's measurement to ensure that even unused Transcended Units can contribute to the battle and stay relevant, if possible even attractive enough to create just to unlock the specific Effect.
However, due to the difficulty for lower ranked players to obtain the necessary Units for an effect, combined with the wide variety of the displayed effects, the disparity between top and low ranked players increased.
The effect's strength increases when all Units required for that effect reached the respective Enhancement level. For example, to unlock "Between a Lover and a Friend"'s effect and its MAX bonus, Transcended Storm Bear, Manalandy and Arrow Trigger need to be MAX enhanced.
Effects are active for all Transcended Units during a battle (regardless whether they were needed to obtain the effect), at all times, in all Game Modes.