Sliding element can be driven by either leadscrew or belt.
There is a balance between amount of friction and play in a sliding element on the rail. It is regulated by the adjustment screws on the sliding element.
The belt drive is faster but weaker, so the rail should slide out of the element under its own weight when in vertical position (18" or 24" rail). There may be slight play when twisting the sliding element, but it can be reduced by fine tuning.
The leadscrew drive is stronger and can tolerate more friction, so moving the sliding element should have cutting-through-butter feel to it. There should be no play when twisting sliding element with a reasonable force.