Any number of polylines/polygons can reside in the same file. Each one begins with a single line header that is used as the "ID" of the polyline/polygon. Next comes the data in either 2D or 3D format, the importing automatically handles either. Finally, the polyline/polygon is finished (not necessarily "closed") by an "*" in column 1. There is no difference between polyline and polygon in a P2D file, only how the data are treated by the application reading the file.
Example:
