REAPER's learn system is lacking the means by which to set a minimum and maximum parameter control limits. if your parameter is 0-100, you cannot specify a sweep between limits 45-90. if you attempt to perform this with a controller, you cannot do so reliably without overshooting and undershooting these limits. likewise, the ability to invert the control is unavaible in REAPER.
these functions are helpful to create tight, performable controller manipulations. you see them in ableton, energyxt, i assume others as well.
check out Resolume's midi learn system:
Learns (cc or OSC): set min, max, invert
Threaded Mode