Split planes allow you to define more than one copper plane on a single layer, rather than the more 'traditional' method of dedicating a whole layer to a single power or ground plane. You may need to do this if your design uses multiple power nets, or perhaps if you need to add an area of ground shielding under specific components.

Split planes can be created at the start of your PCB design process or you can add them as required at any time while creating the PCB.  To add a split power plane, you must add a copper pour areas to a layer intended for a power plane, but it should not be explicitly defined as one.  The power plane layer should be defined with a bias of  'No Tracks' on the [Layers] tab of the Design Technology dialog.

The copper pour areas should be linked to the relevant net by specifying it in their properties but should be left unpoured during editing.  Pouring should be the last stage of editing before final checks. As long as the copper pour area is linked to a net, any autorouter will treat it as already present and not create unnecessary tracks duplicating the pour connection.

The line style of the copper pour area defines the minimum width used by the fill, so it controls how narrow a gap can be filled, taking into account the design clearances to shapes.  When plotting, all areas, including copper pour areas are treated as construction lines and cannot be plotted.