What does the yellow car AY below the license plate mean?

The number segment of the tourist bus.

On the streets, citizens often see tour buses with yellow license plates. Starting from January 22, 2015, these more than 1,600 yellow-plate tourist buses began to uniformly change their license plates. It is expected that in three months, all tourist buses will use yellow license plates starting with "Shaanxi AY". The Transportation Management Office stated that unifying the number sections of tourist buses can further standardize the order of the tourist passenger transport market and crack down on illegal operation of tourist vehicles. More than 90% of tourist buses use yellow cards.