Custom Load functionality is devoted to situation when you should perform a co-simulation. That is to say if you have additional differential equation(s), and you want to integrate them together with the Mechanism ones using the same integrator. Such task often arises when you deal with Mechatronics having electrical/magnetic differential equation in addition to the Mechanism ones.
If you do not have additional differential equations, using Custom Loads is an overkill. If you additional load can be represented as a function of positions, velocities, accelerations and even other forces, you should simply use User Defined profile in Force Motors or Forces/Torques as needed. No matter how complex is this function, the functionality will process it as needed.
