Any number of polylines or polygons can reside in the same file. Each one begins with a single line header that is used as the "ID" of the polyline or polygon. Next comes the data in either 2D or 3D format, the importing automatically handles either. Finally, the polyline or 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
------------------------
Polyline Test
609115.69390674 3643612.72035394 0
608828.057738002 3642922.39387814 0
608569.186338242 3642232.06904821 0
608396.604307826 3640908.94563456 0
...
...
...
601493.350247946 3628713.19503985 0
601464.586301899 3627763.99747289 0
601522.11337106 3627016.14485373 0
601522.11337106 3626642.21854415 0
*