one of the documented procedures in this installation of the ACS
chat_room_group_id   chat_room_id
What it does:
If a private chat room, returns the ID of the group that can enter. If the chat room is public, returns empty string. Memoized for speed
Defined in: /web/philip/tcl/chat-defs.tcl

Source code:

    # throw an error if the argument isn't an integer (security since
    # the memoize will do an eval)
    validate_integer "chat_room_id" $chat_room_id
    return [util_memoize "chat_room_group_id_internal $chat_room_id" [ad_parameter RoomPropertiesCacheTimeout chat 600]]