When a player calls the Arbiter, or when an Arbiter pauses the clocks, it is possible to chat via a chat window.
Either player or the Arbiter may communicate through this chat function via text chat. Both players and the Arbiters will see the chat, it is not private between one player and the Arbiter.
Simply type into the box provided and click Send (or Enter key).
Take note that only a maximum of 160 characters can be used per message. See more about the Arbiter call feature here.