Hi, sorry I missed this when posted.
The reply is rather complicated, depending on lots of other factors and how you are designing your board.
However the basis is that you would use a coper pour template after the board has been routed to then allow you to pour a full copper layer onto an electrical layer to use as a gnd plane.
Or when setting up your board to define areas that make up a split plane, perhaps with lots of different voltages on so that in the routers you can select them and pour copper.
Or on routing layers, for after you have routed to add additional copper areas.
You do NOT use Maximise copper.
That is a feature that was available in the DOS version and IMO should no longer be in CADSTAR however some older users are likely to be unhappy with it's removal. Pouring with templates is far superior.
(You are welcome to try it, laugh at the results and then undo it all.)
IMO you need to scour the help files when doing this to fully understand what is happening.