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The 5e PHB says a Rogue can only apply Sneak Attack (given the conditions are met) once per turn. Does this differentiate between the Rogue's turn and the entire combat round? If the Rogue gets to make an opportunity attack as a reaction, and the conditions are met, can he apply Sneak Attack on that? Is that considered a separate turn? Does using it on either one prevent its use on the other in a single round of combat?