Non-lethal damage

Subduing an enemy via non-lethal damage

RAW: "When an attacker reduces a creature to 0 hit points with a melee attack, the attacker can knock the creature out. The attacker can make this choice the instant the damage is dealt. The creature falls unconscious and is stable."
Ruling: As RAW, with the caveat that it's DM discretion on whether your character can determine what would be non-lethal (for humanoids and animals your character probably has a pretty good sense of it, but for oozes, golems, undead, and other creatures with unusual systems of "life"... less so.) However, when possible, I would prefer to know ahead in clear terms that you're intending not to kill someone (if only so I don't start narrating a death blow and have to backtrack).