Aspects that affect HITL

I am working on an HITL simulation of custom VTOL and Fixed-Wing airframes, and I was wondering what all goes into an HITL simulation.
My current understanding is as follows:

  1. The flight control - Sensory perspective: The aspects of the flight that take care of understanding how the craft’s attitude, orientation and system performance is handled by the Flight Control Hardware (Pixhawk FMU v2.4.8 in my case) in the HITL mode. The same is handled by simulated sensory environments in SITL. In both SITL and HITL, feedback is taken from the simulation environment (Gazebo, JMAVsim etc).
  2. The flight control - Actuation perspective: The aspects of the flight (in simulation) that require actuation, such as the motors and control surfaces, is what I currently am unsure about. While writing the SDF files for a simulation model, we add meshes that provide visual representations of the mechanisms (rotors, props, fuselage) that we use, and add plugins (lift, drag, motor model) to describe how they work. Do we need to adjust the plugins to reflect the properties of the motors/aerofoils our custom designs use? Are these plugins used in both HITL and SITL? Do the plugins determine how much thrust/torque the motors are likely to produce in the HITL mode as well?

To summarize my query, what all would I need to take into consideration when attempting to HITL simulate (as closely as possible) a custom airframe?