Extra Radio Unit and voice chat interface ?

[CougarFFW04]

Hi everyone,

My apologise if this has been already discussed.

I am familiar with the com 1/2, chanel and how to use them with the PTT buttons
BUT what is the purpose of the Extra Radio Unit and how to use it ?
Is it the way to chat with players in the interface before entering 3D cockpit ? If not how to do that.

Thanks