The term VOID-Slab sounds a misnomer. Anyway, it can be a hollow core, waffle or ribbled slab. These are slabs with beams spaced closely with hollow core PC blocks aligned adjescent to the thin RCC beams with nominal RCC deck slab over them, they are usually casted Monolithically. The best way is to make the formwork for the Rib beams, place the Void-Hollow-core blocks, place the reinforcements and cast concrete Monolithically.