A plane in CombineZ has thickness, unlike the geometrical version. When you have a Picture and you press P you will see which parts of the Active Frame contribute to the Picture. This is the Active Plane. Use the Up and Down Arrow keys to change Planes in a similar way to changing Frames. The Active Plane temporarily replaces the Picture, so pressing Tab shows the whole Frame of which the Plane is part.
If you press P while the Mouse is pointing at a Pixel inside the Active Rectangle, the first Plane you will see is the one to which that point belongs. This provides a neat way of going directly to a Frame. Point at a pixel from it and press P, then if you wish to view the Frame, press Tab, remember you will now be in Plane mode so you will probably want to press P a second time to continue working normally.
You can use Planes in combination with other view modifiers, e.g. press D and you will see the depthmap for this Plane. If part of a Plane is difficult to see against the current background, press Space to invert its colour.
Fix, Paste and Clear Rectangles in exactly the same way as with the whole Picture, but now only Pixels from the Active Plane will be modified, although if you Paste in apparently empty places, Pixels from other Planes may be replaced.
Similarly Fix, Paint and Clear Pixels using the Mouse, the same proviso applies as above, and use C to seed Fill areas etc.
See Rectangles for one practical use for Planes.