Note that all dimensions are intended to be maximum. A smaller sail is not "wrong".
And also note that the indicated sails are not by any means the only ones "appropriate"
for a given boat. To make the screens readable we limit the displayed sails to just
those that are likely to be most commonly used. Stormsails, for example, may not
be listed but that does not mean that they are inappropriate or unavailable.
These dimensions are intended as starting points only. Sailboats are not all made at the same time or in the same plant with the same materials -- even when they have the same designer. So carefully measure your boat if there is any uncertainty.
Sail dimensions that are not followed with an asterisk are either based on class rules or designer blueprints or measurements of sails. Though highly reliable, they should be checked.
When the sail dimensions are followed with an asterisk, the data is "calculated" using a computer algorithm that makes assumptions where necessary information is lacking. For example headsail dimensions are based on the assumption that the J and I dimensions form sides of a right triangle. The maximum luff length is calculated using this assumption. So is the angle of the forestay at the tack. Then a "deck angle" is assumed to calculate the height of the clew and, thus, the length of the leech and the foot. This angle defaults to 4 degrees for jibsails, 9 degrees for genoas, and 12 degrees for drifters. This generally results in an acceptable clew
height, but it can and should be modified to make fairlead locations work. Mainsail and mizzen leech dimensions are based on the assumption that the tack angle is 88 degrees.