Standard rate.
There used to be a thing called the standard rate of descent. It means that you can descend at whatever ROD you like but the last 1000 feet have to be at 500 fpm. That is where the 500 fpm comes from, though this method is not used any more. Today one should always aim to climb or descend at a rate of at least 500 fpm because the TCAS will only give a climb or descent arrow if the aircraft is doing at least 500 fpm. This seems to be the smart thing to do now days. As far as an official ROD goes, you're the man flying the plane, if you need to slow down do what you want and don't worry about being bullied by ATC into minimum RODs.