I think that Tom, W5KUB has summed it up well.
There are a lot of different ways that it can be connected and configured on some radios.
And speaking of Tom, purely by coincidence I talked to Tom on 40 meters one afternoon last week.
I use a K3/KPA500/KAT500 with the RigPi and the connection is Analog for audio using 2 3.5mm cables between the radio and the RigPi.
The CAT control from the RigPi is via a FTDI USB-DB9 cable to the radio.
Here I have also used a TS-590S using only a single USB A-B cable.
In addition, I have used the FT-991A with a single USB A-B cable.
73 - Mark - K8MHW
'A RigPi Enthusiast!' - I am not otherwise affiliated with MFJ or RigPi