Heater and a/c blend?

[Sawlog]

I just bought a 1994 Chevy 3/4 ton 4x4.
On the heat and a/c controls, it has a blend up and down botton and a defrost and floor button next to it.
On this truck I can have 0ne of three functions. Defrost, vents or floor. The blend switch just seems to move the air to the next position up or down, I'm assuming that I can't have air at more then one place at a time? Like part defrost and part vent. Part vent and part floor?
Thanks, I hope I explained this right.