Your sign plays on what we refer to as a channel. Now, let's say that you have a chain of stores with one of them being a headquarters. Each store will have its own channel, playing content that is appropriate to that location.
Periodically however, you may have promotions or other communications that you would like to easily add into what's playing on your satellite store's signs.
That is what the channel mix-IN feature is for. It's for injecting specific pages of information into the playback sequence of your other store's signs, without you having to manually do so on each and every sign.