This is a counter made with 4 wired counters. When a counter is over "F", it send a signal to the next counter to increase out decrease 1.
The carry wire is used to increase 1 for the next counter and the borrow wire is used to decrease 1 for the next once.
If you want more information about counters, follow this link: http://everycircuit.com/circuit/5320201688055808/counter.-documentation-needed-.
