The area of the foot that is directly on top of the arch. The most exact way to measure instep height is to make a circumference measurement of the instep with a tape measure, and compare that to the length of the foot.
Standard - The circumference height is less than 1/3 the length of your foot.
Medium - The circumference of the instep is approximately the 1/3 -1/2 length of your foot.
High - The circumference of the instep is greater than 1/2 the length of your foot.