Hello A/C, I don't know the specific equipment you are working on, but this is from my experience.
The acft power is actually a range, usually 110-120 vAC, 3 phase, ABC rotation, 400 Hz. The AC electrical equipment used this as needed, some only need one phase, some all three. Radios and transmitting devices often need both AC and DC power to operate normally. The valves and electric pumps on acft are also designated for specific operating qualities, and require both.
An AC powered motor has low starting torque, high HP, and more efficient power use, but is generally a uni-directional motor. In order to vary speed and direction, you have to modify the phase and rotation sequence. These are most often found as fuel and hyd pumps.
DC powered motors have much higher starting torque, but are less efficient and generate more heat. However, simply varying the voltage and polarity in a switch can change their speed and direction. These motors are most often found in linear actuators and valves. The additional factor to a DC motor is battery or emergency backup.
Most engine ignition and starting valves are also DC. This provides additional battery capabilities for starting at outline bases and emergency air starts when AC power may not be available.
Most GPU's use a 6 pin receptacle, AC phase A, B, C, and ground, plus a two "safety pins" that are closed when connected to an acft to tell the GPU its safe to close the contactor and allow AC power to the Acft. The DC power is then supplied through TR's (we talked about them earlier). Smaller Acft often use an "E" pin connector, which is DC. It has three pins, +, -, and safety contactor. This usually provides enough power for the instruments and to start the motors. The required power type is based on the generation capabilities of the on-board generators, and so dictates the required power on the ground.
I don't know why 115vAC or 28vDC was chosen, but it has become the airline standard, and my acft calls the DC busses the "Isolated, Essential, and Main DC busses",
so I apologize for not answering your question specifically, but I hope this helps!
"Never trust a clean Crew Chief"