A portal (.prt) file is a list of the portals in your .BSP, generated by VBSP and then used by VVIS to perform visibility calculations. It is a simple ASCII file and its format is as follows:
PRT1<Leaf Count><Portal Count><Vertex Count> <Leaf A> <Leaf B> (X Y Z) (X Y Z) (X Y Z) (X Y Z)...<Vertex Count> <Leaf A> <Leaf B> (X Y Z) (X Y Z) (X Y Z) (X Y Z)...<Vertex Count> <Leaf A> <Leaf B> (X Y Z) (X Y Z) (X Y Z) (X Y Z)......
The PRT1 signature is specific to Source, older tools don't generate it. You can view these files in GLView or 
Hammer.
