What is required in the "vetting" process? When can I require prepayment?

I have been renting my unit on other platforms for many, many years. I have 40+ 5-star reviews. I only have a few bookings on BOOKING.COM because it's not very popular in the US. 

I require pre-payment to rent my unit. Booking.com has a mysterious and undefined "vetting" process before they allow pre-payment. 

What are the requirements to be fully vetted? One of my properties is fully vetted; one is not. 

Please advise. 

Also, this is what the help article says, and it doesn't make sense:

When will this restriction be removed?

The “No Prepayment” restriction will be lifted once you’re by one of our teams.