Bus types?

[pengyou]

I have seen some websites that describe type D buses as being transit buses - use the term interchangeably. Are they the same? I have always thought that transit buses usually had a higher interior height, often were a little wider and had no space under the floor....but all of the city buses (transit, to my way of thinking) all had 3 steps to get up into them (meaning that there is probably space under the bus). What gives? I would love to find a 35+ft long transit but that has space under the floor