Minecraft wiki entry for Command_blocks_and_functions, associated text: Other than the ninja, it's a straight tour of the 5 tiers of swords and armor. Note that the Weight tag, used right after the Entity tag in each SpawnPotential. That determines how rarely it's picked. Specific numbers don't mean anything, just the ratios between the numbers. For example, if you want all the potentials to have an equal chance just set all the weights to 1. Changing them all to 8 wouldn't do anything different (they're still all the same size as each other), however, changing all but one to 8 would make that one 1/8th as likely as the rest. In this code, 4 is the default. The ninja only has a weight of 2 (making him half as likely to be picked) and fake Herobrine has a weight of 1 (making him 1/4th as likely to be picked, and thank god!)