The 5V supply is provided from a separate header, not connected to the carrier board which is required for full transmit power (30dBm) use.
The SBUS/PPM I/O cable is 3.3v tolerant and is able to be a source or sink of PPM or SBUS, while also mixing formats on either end.
** The orientation of the cable is such that the black triangle is towards the PIN 1 label on the 16w header on the RFD900x PCB. The red wire is on pin 4 of the modem. Inserting the cable the incorrect way, can cause damage to the modem.