I'm still guessing it is a pressure switch and the Tee'd connection plugs into the wall 24V transformer and one into the pump.
You can always experiment. Run some RO tube from your RO unit (anywhere that is easy to access and you get water) into the switch, then from the switch to the pump. Then on the output from the pump, put more RO tubing in so you don't make a mess anywhere. Turn on the booster pump and plug the output with your finger. If it is a pressure switch, then your pump should turn off.
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