How do you decide the difference between premium RFPs and non-Premium RFPs?

It depends, but mostly it's the budget.

The primary difference between premium and free RFPs is budget. For almost all  premium RFPs, we've been able to vet and confirm the budget.

In some cases, some RFPs are also marked as premium and are likely to attract a high degree of interest, but they are moved to premium to  reduce competition.