major v. minor scoring
Welcome to the world of competiton shooting!
To your question, in USPSA, there are scoring zones that are assigned point values on each paper target.
The same scoring zone can have a different point value based upon whether the round is categorized as 'major' or 'minor' power factor.
Zone Major Minor
A 5pts 5pts
B/C 4pts 3pts
D 3pts 1pts
Theoretically, the 'major' load is more difficult to shoot quickly, hence it earns more points per shot. 'Major' caliber is .45acp, .40s&w, 38super (there are exceptions to this), and 'minor' caliber is 9mm. (After a bit of practice, I found that I shoot my Glock 35/40s&w as quickly as my Glock 34/9mm...you get familiar with the recoil impulse and the way the sight tracks and you simply don't notice the difference).
IMHO, there are more important considerations to make regarding the selection of a competition pistol than the 'major/minor' power factor rating.
1. Do you already have a pistol of 9mm, 40s&w, 45acp caliber that you like to shoot? If you do, I would suggest that you try a few USPSA matches with your current pistol, just to see how you like it. At the match, you'll see many people with different kinds of pistols, most are happy to tell you 'more than you'll want to know' about why they chose that particular pistol/caliber. Most will let you 'try out' their pistol/gear as well. It will give YOU a good idea of how the sport operates, what type of equipment YOU would like to use or try, which Division YOU would like to play in, etc.
2. Do you reload?
If not, 9mm is much more inexpensive to shoot (obviously translates to more shooting).
3. What's the size of your gun budget?
Probably the most inexpensive Division within USPSA is the Production Division. As the name implies, pistols in this Division are mostly 'off-the-shelf' pistols from Glock, CZ, SigArms, Springfield, etc. Usually inexpensive pistols that run very well with inexpensive factory 9mm ammo. Other pistol Divisions provide for more expensive options.
4. Do you plan to use this gun for home-defense/carry as well as competition?
If so, Production division is probably the choice to make.
In any case, pick the pistol that you like to shoot the most and get to a match with it.
Whichever equipment Division you participate in, realize that you are only competing against others within that same Division (sort of a 'match within a match').
Take a peek at the
USPSA website and review the "How Do I Get Started" and "What is Practical Shooting". It will give you a good basis for understanding the different Divisions, equipment, scoring, etc prior to your first match.
Wish you well in your endeavor. It's an absolutely great sport!