TL;DR: When we name areas, the surrounding lines are automatically named as well. the top and left lines for the "header" area will have the name of "header-start", whilst the right and bottom lines will have the name of "header-end". Areas can also be automatically named by naming the surrounding lines with the "-start" and "-end" suffixes.
Source: https://www.smashingmagazine.com/2017/10/naming-things-css-grid-layout/