You need a data axis, when you want to add a grid-based series to a region, see Basic concepts for more explanations. After adding a data axis object to a region you have accsess to it for setting all needed properties which belong to data axis objects. Each region can have one or two data axes, therefore you must pass an Index when you want to get access to a previously added data axis.
To add a second data axis, just call this method twice. The second data axis can have the following properties different from the first added axis: Alignment (this one must be different from the first axis), AxisText, AxisUnit, MaxValue, MinValue and DecimalDigits.
You are not forced to add a data axis to a grid. If you omit the data axis, the grid will be painted without axis lines and without labels. But: In this case you can't set MaxValue and MinValue: dynamic calculation of these values takes place.