Public chats can be viewed by any FMChat user, not just by the people "chatting" on the chat. These are great for chats you want anyone to be able to follow along with, or for general questions posed to the whole team.
How do I create a public chat?
When starting a chat you have the opportunity to tag it as public right on the "Start Chat" tab: you'll see a checkbox at the bottom of the screen beside the "start..." button.
Existing chats may not be set to public. (What would happen if someone could take something that was previously private and suddenly make it public?)
"isPublic" is also an optional parameter when creating chats vis scripts using one of the Chat APIs