Even if you order a stock, a template will be useful to cut the stock piece to the right size of your hairloss area.
Also, remember that a template takes into account the dome and curvature of your head, not just the area size.
Some people with an irregular head size/shape might find a stock piece doesn't fit as well as a custom piece will.

A, AA, B, BB etc are the various shapes /contours for the front hairline.
They range from a straight hairline to a receded one.

But, although these initials are used by all companies, they are not universal.
So a 'C' contour at one company won't necessarily be the same shape as a 'C' contour with another.