Topic is a game mechanic in Tokyo Mirage Sessions ♯FE.
It is similar in functionality to the Support systems that exist in other Fire Emblem games. Represented in the game as an instant messaging service, it is used by Itsuki to exchange messages with his office friends in real-time. The player can advance the story through these messages, with messages related to the main story and side-stories separated into their own tabs. On a few occasions, the player is given a choice on how to make Itsuki respond to a comment or question, though the choices presented don't affect the story in any significant way.
The player views these messages using the Wii U's Gamepad. When a message is received, a notification sound plays and the Gamepad vibrates. In addition to messages from the other characters, Itsuki will also receive special notifications over Topic when new Carnages and Radiant Skills are ready for Unity.