From what I see in the code, it's just the chat channel you mentioned as the "second hidden channel". I'm not seeing anything that references the community system.
If you want to take a look at it yourself, the function responsible for sending messages is at line 349 in Channel.lua.
The API function that actually sends the message is called
SendChatMessage().
Here's a breakdown.
Code:
SendChatMessage(segment, 'CHANNEL', nil, self.number)
- segment - This is the "message" the addon sends.
- 'CHANNEL' - This is the channel type the message is to go through. 'CHANNEL' specifies a numbered chat channel as opposed to say/party/raid/etc.
- nil - This spot usually specifies a language for a character to speak for a supporting chat channel. 'CHANNEL' doesn't support this, so it's left blank (what nil signifies in Lua)
- self.number - This is the chat target. In case of 'CHANNEL', this is the channel number to send the message through.