Namlakhorn there is an airside Arrivals lounge which is shared between First and Business class passengers and Al Maha arrivals service. The service is almost the same, Al Maha or not. The Al Maha services are described at http://www.qatarairways.com/us/en/al-maha-services.page.

If you have been given STPC then the Al Maha arrivals service is almost certainly a waste of time. If you are in transit, then upon return to the airport and you have your onward boarding pass then you would bypass check-in and go straight to immigration/security. Again Al Maha departures service is probably useless for you.

Remember if you are in transit you will not be allowed to access tour check-in luggage and you should have everything you need in your hand baggage.