Despite there being a wide range of qualifications and standards that applied by government agencies, most these I don't consider particularly relevant to self-defense.

In order to reliably stop an assailant you have to get good hits on parts of his anatomy that matter in the immediate situation - like the vasculature of the heart, or the brain. While punching holes in his digestive tract might be fatal or debilitating in the long term, there isn't any physiological reason for them stop right now.

In speaking with my "personal physician" (former SF Medic, Currently an ER Physician, and SWAT team member) he explained that if you were to pull an assailants still-beating heart right out of his chest, there is no medical reason why he could not continue to fight for another 4-5 seconds.

This knowledge impressed upon me the need for getting good hits. Short of shooting him in the brain, the best hits I can deliver might still give him a few seconds of action, and anything less vital than that is could be irrelevant to the situation at hand.

People get confused by shooting big dumb targets like the B27. New shooters look at their performance and note hits to the shoulders, grazes of the neck, and gut shots with satisfaction. They ignore that the very posture of the B27 is an unlikely position for assailant to be in. As soon as you start using photo realistic targets, with crouched or bladed asymmetrical postures it becomes apparent a higher standard of marksmanship needs to be applied.

The assailants vitals do not get bigger or smaller depending on the distance. It is easier to align your sights on a closer target than a distant target, but the target itself remains the same size. Everything worth shooting is about the size of a large clenched fist.

This is not to say that a high level of accuracy is required for self-defense, but that without a high level of accuracy the assailants continued participation remains optional. If he is unimpressed with your marksmanship or choice of caliber he may choose to continue the assault. Poor hits discourage him for continuing the attack, but do not force him to quit.

Speed and distance are also factors effecting accuracy. It is important to know how long it takes you shoot a fist size group at a given distance. As a personal training note as soon as you can consistently shoot a fist size group in whatever you believe to be a realistic time, increase the distance.