I think it all depends on how things are written in the owner's manual. The way I understand it is if your vehicle has 4WD, full-time is when you shift it into that position and all four wheels are driving no matter what the surface. Some 4WD vehicles have a part-time that when not in full-time only one wheel drives unless it loses traction and then the other wheels will lock into drive. My Escape doesn't have a way to shift into full-time, but has what some manufactures call positraction - when the drive wheel slips, the other three lock into drive. I think Ford calls it on-demand 4WD or something like that. I have no idea what 4WD Low is.