Or more commonly known as a CALS file. It is a file formate that was developed by the US military. The reason they are commonly used is it that they are scalable, your can even measure them and do area take off's.
CALS file have actually become a stand archiving file format here in Sweden. I think that AutoCAD supports it of of the box, though I could be wrong. It woks much in the same way as PDF, in the seance that it's a virtual printer.
Cheers.
Ben
