Underbody paint is used to protect the various components exposed to road grime. The same is used on the wheel well and it's also used as protective wheel arch paint since the arches are also exposed to the same hazards.
https://aussielife1.mystriking....ly.com/blog/underbod

Underbody Car Paint: Protect Your Wheel Arches
aussielife1.mystrikingly.com

Underbody Car Paint: Protect Your Wheel Arches

The most exposed and least protected side of any vehicle is its underside. Despite housing some pretty important systems and components, the undercarriage, as it's also called does