Per the IBM documentation:
In an n-way chat or a meeting, the location of the server on which the chat is logged is the server on which the Place is handled.
In an instant messaging session, the chat is logged onto the home server of the user that accepted the IM.
Note If user A initiates a chat with user B, the chat is logged in the home server of the chat’s recipient – user B. If in the course of the chat, user A closes the chat window, but user B continues to write, a new chat session is created in which user B is the initiator and user A is the recipient. From this point on the chat is logged on the home server of user A.