I'm having trouble deciding weather to use standard loads or major loads on my P14 every time I go to the range. What is the difference between them and what are the things that I should consider when using either one of them during sessions.
Unless your shooting bullseye type target loads in your P14, the standard loads are major loads. Major is simply short for Major power factor, used in USPSA, which is bullet weight in grains times velocity divided by 1000 equals power factor. Anything over 165 in Major power factor, anything from 125-164 is Minor power factor. It makes no difference for range use in a 45.
In a .45, IPSC "major" loads are poop loads. Even from a 3" barrel, standard 230gr hardball will make the old 175pf, so a 165pf load in a 5" gun is well below normal power levels. If you shoot IPSC or IDPA and want to practice with your competition ammo, then loading down around 170pf would be good for practice. If you want ammo that simulates defensive loads (in terms of recoil, muzzle blast, point of aim/point of impact), then load up to full .45 ACP power (185-195pf)