I know this sounds really confusing, but I would say the second paragraph above should answer your question. If you both (bi axial moment) then you need to check all the angle to make sure your moment is within the PM curve of all the angles. In short, if you have just a moment on your column in x axis then you need to check 0 degree result and if you have moment in only y axis then check 90 degree PM curve. In that case the cutting plane is not a horizontal line, but it will be a line drawn at 20 degree with horizontal and the rebar at ends of this line will be in compression and tension. Next it will increase the angle to next interval (for example 20 degree). At that time the rebar on the right side and left side will be in maximum tension and compression. The first curve will be generated for the angle = 0 (horizontal). Etabs will generate a 3D profile of PM curves. We design the moment frames using PCA Slab. We design the piers of the shears walls for flexure and axial loads using PCA Column. Once we are able to limit the drift, we can output the forces from ETABS into a spreadsheet for design. For example, if you have square concrete column. ETABS is used for the analysis of concrete shear walls and concrete moment frames.
In ETABS the PM curve is generated for the each cutting plane of the member in the cross section.
