Permanent trailers -- what are they when they aren't permanent?

This morning while driving my wife to work, as I moved into the center left-turn lane, I came to rest briefly behind a large transportation truck -- sometimes called a "semi" or "semi truck and trailer." The beast had a sticker near the license plate that said: "Permanent Trailer."

Being a man of words, this phrase hit me as very odd. What, I wondered, is a "permanent" trailer? What would it be if it weren't permanent -- temporary? Are there "occasional" trailers, like there are occasional tables? If one is occasional, what is it when it isn't a "trailer"?

• a "parker"?
• a "get in your way-er"?
• a "take up space-r"?
• a "stage on wheels"?
• a "mobile aircraft-lander", perhaps for very small planes, helicopters, or Harriers?

The list of alternatives could be huge. Anything come to mind to you?

I'm sure this phrase makes sense to those in the transportation industry, and perhaps someone from that group could let me in on the secret. Or maybe you have some good guesses.