Headroom is the space between the top of the door opening and the ceiling.
Vertical supports (or stiles) span across the width of the door providing stability while the door is being opened.
Nylon rollers are more expensive, and don't last as long, but are much quieter. Steel rollers are less expensive, last longer, but tend to be noisier.
The cycle life of a spring is the mathematical calculation of how many times a spring can be wound/unwound before breaking.