It really doesn't matter which handle works which function as long as YOU know and can operate it properly.
Drive the spreader around empty to watch and learn how it operates, and which lever and settings looks right.
Being a ground drive spreader, you must disconnect the mechanisms (both floor chains and beaters) when you are driving from the barn to the field where you are going to unload the spreader. No need to have the beaters engaged if the load is not moving backward in the spreader to be spread. Even more Critical...if you leave the floor chain engaged as you move to the field (and the beaters are not spinning) it will jam the load into the back of the spreader and JAM or break the beaters....or make a dandy mess on your driveway.
So 1) disengage both floor and beaters when not in the field. 2) do not fill the spreader full in the back to the point of jamming the beaters...you want the load to gradually move back into the beaters. 3) Try some different settings for the floor. Start with the slowest movement first, see how that works, if too slow move to a faster setting. Eventually you will learn which setting is right for the types of loads and fields you have. Once you understand what works best for your situation, you can just use the same settings every time you spread.
I am sure this is the basic instruction you will get in the manual... when it arrives. In the interim the barn needs to be cleaned. So 1) be sure the spreader is totally disengaged when driving to and from the field. 2) don't over fill the area near the beaters 3) experiment to find the best settings and ENGAGE both floor and betters when spreading. 4) disengage when finished spreading and going back to the barn. 4 important steps.