what's mean for the split prism on the eyeglasses prescription?

[online glasses]

Such as: 8^BU O.D. would be split 4^BU O.D. and 4^BD O.S. But when splitting horizontal prism the lenses would be both the same base direction (both lenses "base in" or "base out") to create the compounded prismatic effect. Such as: 8^BI O.D. would be split 4^BI O.D. and 4^BI O.S.