Every font or typeface has its own construction. Apart from basic elements such as baseline and x-height, each font has a cap line.
Cap height or cap line is simply the distance between the baseline (bottom line, on which the letters are “sitting”) and the top of capital letters for a particular font. Standard cap line counts for flat letters such as H or I. Round letters like S and pointed letters like A usually have a higher cap line and slightly extend the standard cap line thanks to overshoot, as it allows to achieve the same size effect and bring visual balance to the text.
Some letter elements like ascenders normally do not extend the cap line. However, there are some script style typefaces that have overshoot ascenders and in some typefaces ascenders can appear below the cap line.