What is the purpose of this PLC circuit?
EDIT: added a little more information and made it more accurate.

BTS4140 is a a high side load switch that consists of an active low input, power supply input and output terminal. A load would be connected to output with power supply connect to Vbb terminal. When input pin is pull low, the internal control circuit which consists of a charge pump, will produce a voltage on gate of internal mosfet that is sufficiently higher than output terminal (essentially the source pin of that mosfet). Doing this causes the internal mosfet to turn on with practically low voltage between its drain to source terminals thus connecting Vbb to Vout. 

output protection circuit  is a RCD snubber circuit aka a RCD clamp. Though by the looks of it it isn’t drawn right. 
That capacitor, resistor, and diode form a circuit that will safely clamp (ie limit)  inductive kickback in the case that load is inductive. That is When the internal MOSFET turns off, the RCD will prevent voltage across drain to source terminal from rising to a level that will damage MOSFET.  Diode ensures unidirectional current flow, R dissipates energy, and C stores energy. Usually though, R and C Are in parallel and then connected in series with diode but it would all be reversed with anode of diode at ground. 

Source: https://www.maximintegrated.com/en/design/technical-documents/tutorials/8/848.html

http://electronicsbeliever.com/snubber-circuit-design-analysis/