ZBmini L2 circuit

while looking for some idea for circuit, i noticed this zbmini circuit S1 show as red line. is that live line? where the live line came from? connected internally in the cuircit same as Lin?

State Changes of the External Switch

In the MINI circuit, the S1 is at signal ground level while the S2 is hanging with a pull-up resistor at 3.3V level. The change of the external switch state triggers the S2 signal level change. when S1 and S2 are disconnected (external switch state is open), the S2 signal level is still kept as 3.3V high level signal, the connection of S1 and S2 would lead S2 signal level into “ground” low. Here is a table to indicate the relationship of external switch state vs signal level.