The A50 switch is used as the 50 Ohm load for open ports on the HFC3000 combiner to prevent tuning degradation.
When not powered, at rest, the A50 switch is connected to 50 Ohm. By powering it with 12V the switch will connect the input to the combiners output and the combiner will be connected to it’s corresponding filter.
The diagram shows a typical use case when the filter is switched to a monoband antenna instead of a tribander, in which case the combiner input is connected to 50 Ohms.