If a behavior is under stimulus control, it occurs when you ask for it, and doesn't occur if you haven't asked for it when you are in a training or working session. According to Pryor, perfect stimulus control requires the following four conditions, and only when all four conditions are met is your dog's behavior of "sit" truly under stimulus control.
1. The behavior always occurs immediately upon presentation of the conditioned stimulus. (Your dog always sits when you say, "Sit!")
2. The behavior never occurs in the absence of the stimulus. (Your dog never sits if you haven't asked him to sit.)
3. The behavior never occurs in response to some other stimulus. (Your dog doesn't sit if you say "down.")
4. No other behavior occurs in response to the stimulus (Your dog doesn't lie down if you say "sit.")