This is an example USDA result from creating a Variant Set and adding Variants to the Set.

#usda 1.0
(
    defaultPrim = "World"
)

def Xform "World" (
    prepend variantSets = "shading"
)
{
    variantSet "shading" = {
        "blue" {

        }
        "green" {

        }
        "red" {

        }
    }
}