[RickB]

Divisions are intended to separate guns of different types, so that the competitor, rather than the equipment, is the determining factor in scoring.
Since reloading is such an important aspect of USPSA competition, guns with limited magazine capacity do not compete in the same divisions as guns with higher capacity.
Same for optical sights vs. iron sights.
Your G17 best fits in Production division, but if you want to stoke the magazines to capacity you'll have to shoot in Limited, where the minor-power scoring of 9mm hurts you relative to those "making major" and getting more points on target.
Classification is a measure of relative skill, and you can gauge your progress as a shooter by regularly shooting classifier courses-of-fire and classifier matches.
In my area, there's a classifier stage, at least one, at every match, so if you shoot regularly you can track your improvement through your classification percentage (essentially, how your shooting compares to the very best shooters who'd rate 100%).
Some people fly through the classes, going from D to Master in a year, while others can take years to improve; the more time, energy, and money you devote to improving, the faster and farther you'll go.