Standard messages area includes three tabs (Chat, Q&A, System) - when a private chat is started, it adds a new tab for each private chat.
Chat
Chat (this tab contains the public chat).
Q&A
Q&A (this tab is used for posting questions to other participants and they can write their answers, resulting in a structured view).
System
System (this tab is used for system notifications, e.g. joining or leaving a session, dropping out of the session etc.).
Note: You cannot write into the system tab, it is read-only.
Private Chat
Private chats (each tab represents a private chat with a certain user).
If an unread message is waiting in a tab different from the currently selected, you will receive a notification and the appropriate tab will be colored orange.