A Resistance simulation is similar to an effective horsepower (EHP) towing test in a model tank. The model is effectively "pulled" from a towing point (by default, the CG), and is accelerated from 0 speed to the desired speed. After some period of time, the heave, pitch, and resistance settle out to relatively steady values, and the result is an estimate of the heave, pitch, and resistance (and EHP). Note that we generally use the terms "heave" and "pitch" to differentiate this dynamic behavior from the static "sinkage" and "trim" that is computed by Orca3D hydrostatics. This kind of simulation can be done at very early stages of design to predict the resistance characteristics of the design as well as to hydrodynamically optimize the design by modifying the hull and appendage geometry. Resistance simulations can include just the bare hull or may also include appendages and superstructure. Streamlines, dynamic pressure, and other hydrodynamic phenomena such as separation from spray rails, etc. may be studied. These analyses are typically very easy to set up and relatively fast to run, and will yield an estimate of EHP. This can be useful when comparing one hull to another and even as an estimate of the necessary installed power if you are confident in the relationship between installed power and EHP (accounting for efficiencies such as open water propeller efficiency, hull efficiency, relative rotative efficiency, and mechanical efficiencies).
One limitation of a Resistance simulation is that because the model is being towed from the CG, the steady state heave and pitch (and therefore resistance) may not be the same as if the model were being propelled by thrust along the shaft line. This can be mitigated to some extent by moving the tow point up or down, to the point where the shaft line intersects a vertical line through the CG. Another limitation is that the modification to the flow field around the hull due to the axial and rotational flow induced by the propeller (e.g., wake and thrust deduction) are not accounted for, since the propeller is not present. This is less important with outboard than inboard propulsion systems.
If the geometry of the propulsion system is known (e.g., shafts and struts, lower unit geometry) and the propeller information is known then you can consider performing a Self-propelled simulation. This has the advantage of applying the thrust at the correct location and in the correct direction (along the shaft line), and simulating the axial acceleration and rotation of the flow due to the propeller which will affect the hull performance. But it is important to remember that the propeller is being approximated by an actuator disk, and usually the actual 3D propeller geometry is not part of the simulation (of course it can be, but that requires the exact propeller geometry and is a more complex simulation that takes longer to run and requires the Premium version). The actuator disk imparts momentum to the fluid (axial and rotational), based on the traditional propeller open water performance data, Kt and Kq vs. J values. If you know Kt and Kq as a function of J, you can simply enter those values. That might come from the propeller manufacturer or another software tool such as NavCad. Otherwise, you can enter propeller parameters, and Kt and Kq vs. J will be estimated based on either the B-series propellers or Gawn-Burrill series propellers.