We are all trying to be as efficient with our time as possible. If you receive an Inquiry from a Premium Client, you should assume that the Client is more qualified that an Inquiry from a Basic Client. Premium Clients have paid a subscription fee in order to increase the likelihood that they will find a good Expert. That means that they are serious, that they have budget, and that they may very well be looking for multiple Experts.
Of course, Basic Clients can also be serious about finding Experts. With Premium Clients, you can be sure.