What are detail sprites? - They are the little bits of grass that automatically appear on some materials when used on a displacement. Which ones show up depend on what is specified in the worldspawn entity, which is accessible from "Map Properties" in the Map menu.
This page will show you what detail files the letters represent, and what the sprite materials look like.Most of the time you will use matching pairs but a few maps have one custom and not the other, as follows:
Detail .vbsp files:
Detail sprite materials: