My first question is - why planar milling? If you have a solid, then I would try floor_wall first, and only use planar milling as a backup. Planar milling is not something you can learn in a couple responses, but we can probably get you started in the right direction.
Planar milling is what we used before we had solid models. It is a legacy processor, based on planar regions, defined by boundaries at specific heights. These regions define defined the volume to be removed, or the faces to be profiled, on a prismatic part.
Part is the material to be left when the cutting is done.
Blank is the material before the cutting starts.
Check is material to avoid
Trim is further containment for the path (not the geometry)
Floor is the bottom of the region you want to cut.
For example, a blank boundary would be at the top outer edges of your raw material. A check boundary would be around the top of a clamp. A Part boundary could be at the top or bottom of a pocket. For each boundary type, you define on which side the tool, material, or path resides.
At each level, the system looks at all the regions, and figures out what is machinable, what is not machinable, and where the tool can safely move. Then the system creates tool paths in the cut regions.