This is the description of the Xbox Store from New Zealand.: Description
Create your own custom avatar, then lead them on an all-new quest to bring the Light back to Lucis. Tackle tasks on your own, or team up with up to three other players online! Many exciting features await you in this expansion: an engaging story, an expanded field of play, brand-new quests, and more playable characters—including Noctis and his friends! Membership of Xbox Live Gold for the Xbox One is required in order to use the multi-play mode in the Multiplayer Expansion.
"It gives updates on characters from Kingsglaive and it gives lore to the history of the world and the myths of the Old Kings. The documents and bios explain some motivations and goals for a few characters a bit more clearly than the game did.
The main story for Comrades is you’re a hunter with some memory loss and you’re tasked with gathering items to power up areas around Eos. Along the way you unlock some history of the world and interact with characters from FFXV who provide updates on their life and situation. It’s nice fanservice."