I'm guessing that the +/- is for preload and is used to adjust stiffness of the fork to compensate for rider weight. Suspension forks are usually set up so that there's an initial amount of sag. Increase or decrease the preload to match the specified sag. Unfortunately, I don't know what amount of sag is specified for that fork.

I did some checking and the RST Omega TL does indead have adjustable travel (76mm-101mm). According to descriptions I've been finding, it's a stepless system which probably accounts for the knob.

The adjustments will affect both legs simultaneously.