The expansion adds new content to the main game including an epilogue to the original story, updates to the original game, and the ability for the player to control Aigis. The Social Link system has been removed and the player does not attend school.
Players of the original Persona 3 are given the option of transferring certain data from the original version's save file such as the player's compendium, social-related stats, and maxed Social Link items.
Source: Wikipedia, "Shin Megami Tensei: Persona 3", available under the CC-BY-SA License.
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