POST Load/unload leg by ID

I also see I can update statuses on the leg level by using "Load Leg by ID" as well as "Unload Leg By ID":

Can you provide the scenarios for how to tell if a leg should be "loaded" or "unloaded" (especially for interim stops) ?
Can you provide how entering the different query_params (from_arrive_at , from_in_at, from_out_at, pickup_at) should be used as well as what leg statuses they trigger?