It can be used on any market which Betfair support SP betting
At the time is was created and shared Betfair didn't actually support SP betting on greyhound racing, but that changed a few years ago so it will work on those markets too
The rule re-arms itself every second and also one of the conditions of the rule test the unmatched bets number, its possible therefore that a race could be turned in-play during the second it is taking the rule to re-arm and/or while waiting to receive information back from Betfair you have a unmatched bet in the market