ChannelPort devices can either play clips on the system using the graphic engine to overlay material or only overlay material on video passing through the port. The Channel port uses an A-B mixer internally to choose between two sources.
The diagram above shows how chosen sources are switched between Clip (loaded in the local player), SDI (SDI/HDSDI coming into the input of the player) and Color (internal colour generator). Sources accepted for A/B Mixer inputs are enumerated as follows:
0 - Clip - Decode output: this would relate to the internal player configured on the Channel Port
1 - SDI - External input: this is going to be the SDI/HDSDI input on the Channel Port
15 - Color - Color generator: this is the signal generator for Color on the Channel Port
NB: Please note that a Channel Port doesn't support two internal players attached to the same switcher; to do so, it's required to set up an external player to feed the input with a feed.