Use MUX to make multiple output sockets and to propagate an input to multiple other nodes in the Schematic. You can also use it to clarify both its graphical representation and the connection scheme of nodes within the group.
To access the MUX menu, use:
Timeline, then use ConnectFX (See Creating ConnectFX).
The Modular Keyer, then select a node from the Node bin.
This node accepts a front clip, and outputs a result.