Shaping rules are used anytime you are tracking two different criteria. The first is your current criteria, a behavior your dog can already do reliably. The second is your shopping criteria, a better version of the behavior you are watching for your dog to offer on their own.
In our app, this is most commonly used when adding verbal cues. The current criteria is sitting for a trained hand signal, but you are "shopping" for the moment your dog sits from the verbal cue alone, before you have had the chance to give the hand signal. When that happens, you reinforce it right away. See "Jumping the Prompt" for more detail on this.
A timer is used for these steps. If all reps are completed and more than two-thirds meet the shopping criteria then Push If less than two-thirds of completed reps meet the current criteria then Drop Unless the step has already been repeated then Split Otherwise Stick