Render is applied to perpendicular surfaces. It is applied in successive progressively weaker coats to absorb movement. Each undercoat is scratched to form a key except when underlying a gypsum skim. Its purpouse is to create a plane surface (iron out the bumps)and/or to protect the base wall. It can contain sand, lime, cement, gypsum, additives, fibres etc. Renders are normally about 20mm thick. Top coats can be steel trowelled to a smooth finish but are generally finished with a gypsum skim coat. Thick coats of render tend to fall off the wall or be dragged down and crack. Floor screed is applied to generally horizontal surfaces. It can contain sand,lime,cement,gypsum,grit,additives fibres etc. It can also be a self levelling thin sand/polymer mix. It is usually applied to form a smooth level surface. A sand/cement screed will usually be applied in a dryer state than a render. A screed needs to bond with the floor unless it is a thicker floating screed over say insulation or tanking.