Axial Stacking Diameter

It's the length of the core laminations.

The core of the generator (for any electrical rotating machine or transformer) is made up from thin steel plates laminated (isolated) and then stacked (packed together) to form a big steel pack. If this is not done (use solid piece of steel) high eddy currents will flow and the machine will over heat.

The dimensions of the core pack is determined by the output rating of the machine, i.e the diameter and length. The higher the output the bigger the core pack (more steel) and vice versa.