In construction, "below grade" means below the finished ground level around a building. "Grade" is the surface of the soil after construction and landscaping are finished. Any part of a structure that sits lower than that surface, such as a basement, a foundation wall, or the ground under a crawl space, is below grade, and it's in constant contact with soil moisture. That last part is why the term matters. Everything below grade has to deal with water in the surrounding soil,