This is an example USDA result from creating a Variant Set, adding two Variants to the set, and selecting the current Variant to realistic.

#usda 1.0
(
    defaultPrim = "World"
)

def Xform "World" (
    variants = {
        string shading = "realistic"
    }
    prepend variantSets = "shading"
)
{
    variantSet "shading" = {
        "cell_shading" {

        }
        "realistic" {

        }
    }
}