SwitchLinc A and SwitchLinc B are cross linked to form a virtual 3 way circuit. With the two devices cross linked pressing SwitchLinc A On paddle causes A to send a Group message to SwitchLinc B telling B to turn On. Same is true in the opposite direction. Pressing SwitchLinc B On paddle causes B to send a Group message to SwitchLinc A telling A to turn On.
SwitchLinc C does not react to messages from SwitchLinc A or SwitchLinc B because it has not been linked to either device. This is standard Insteon architecture and has been from the beginning of Insteon.
Now replace SwitchLinc C with a PLM associated with an EZBridge or EZSrve. The PLM does not react to messages from SwitchLinc A or SwitchLinc B because it has not been linked to either device. Now turn on Monitor mode in the PLM. In theory the PLM would react to messages from SwitchLinc A and/or SwitchLinc B without being linked to either A or B. This would allow the application connected to the PLM to be aware of activity on SwitchLinc A and/or SwitchLinc B without being linked to either device. The Monitor feature has proven not to react to messages from SwitchLinc A and/or SwitchLinc B all the time making the Monitor feature unreliable.
The solution that all products are using is to require the PLM to be linked to any device that the PLM will react to. This makes the PLM link requirement the same as any other Insteon device. Just as SwitchLinc C would not react to messages by SwitchLinc A or SwitchLinc B without having a link to A and B, the PLM requires a link to each device it is going to react to. The PLM Monitor mode was a nice idea that did not work in the real world so all applications including EZSrve/EZBridge require the link that was required before the Monitor mode idea came along. With these links in place EZSrve/EZBridge Macros can be triggered by actions of SwitchLinc A and SwitchLinc B providing the control you are looking for.
For a Motion Sensor example, you could link the Motion Sensor to SwitchLinc C if you want the Motion Sensor to have direct control of another Insteon device. Or you can link the Motion Sensor to the EZSrve/EZBridge PLM in which case Macros could be triggered to control SwitchLinc C. You can even do both. Have the Motion Sensor linked to SwitchLinc C so the Motion Sensor turns on C directly and have the Motion Sensor linked to the PLM so that EZSrve/EZBridge could trigger a Macro with a time delay action that turns off SwitchLinc C 10 minutes later.