I have not used the axisymetrical porous media, but have used unidirectional and isotropic. I looked through the help and found the "r" is for the transverse direction and the "n" is for the axial direction. It would be eaiser to understand and remember if they would just write that in the engineering database description. For this type of porus media you need to then define the pressure vs flow characteristics for parallel (axial) and perpendicular (transverse) to the main flow axis of the material. At least that is my understanding.
Hope this is of some help.