From The Truth About Guns:

Force-on-force courses are stressful. You have to be ready for the unexpected without completely freaking out because, just like in real life, your Simunitions firearm comes loaded with a limited number of rounds. Sometimes you get a backup magazine to take into the scene and sometimes you don’t. That meant that under high stress, you had to not only discern what was happening and decide whether to shoot or not, you also had to make your shots count, and make them count in a situation involving many people, or near dark or dark conditions. You would also have to reload in the dark.