There are 2 types of split charge relays.
The first was a relay, wired from the vehicles battery and required a second cable connected to the alternator to trigger the relay when the alternator was charging.
The most comon nowdays is the intelligent relay that doesn't have this alternator trigger wire. It senses when the alternator has charged the vehicle battery to nearly fully charged and then triggers the relay to start charging the aux battery.
I don't know what PF use.