WOW, I thought it was for training new drivers, but found this, because I was curious:
"I recently became a Disney cast member driving the buses. So here is the official answer. We place Jasmine on the marquee when the bus is out if service. Jasmine or Hercules looks better than "Out of Service" to our guests. Training buses are marked "Training" and cast transport buses in guest areas are marked "VIP Cast Member". There are several other marquees that are used backstage such as "Westclock" and "Pluto" which are the MK employee shuttles, "Parade Bus" which transports cast from dressing areas to the start if the MK parade route. If the marquee is blank, the bus is usually full. As you know with Disney, image is everything."