Only one of them can actually talk to the UPS directly. That's where the network comes in. The Master system runs the driver, upsd, and upsmon in master mode. The Slave systems only run upsmon in slave mode. This is useful when you have a very large UPS that's capable of running multiple systems simultaneously.
Some systems have multiple power supplies and cords.
This software understands that some of these servers can also run with some of the supplies gone. For example, if you have 3 power supplies and 3 UPSes, but only 2 supplies must be running at any given moment, the minimum would be 2. This means that you can safely lose any one UPS and the software will handle it properly by remaining online. Sometimes a PC will be close to a UPS that needs to be monitored, so it's drafted to supply a serial port for the purpose. This is not a problem.