What is diffuserin construction?

A diffuser is a supply outlet engineered to spread air in a pattern, typically the square lay-in cones you see in office ceilings. A register is a grille with an integral damper behind the face, common for sidewall and floor supply. A grille is a bare slotted or louvered face with no damper, used mostly for return and exhaust. Estimators keep them on separate lines because the unit costs and the assemblies behind them differ.

Both, and the cross check is the point. The reflected ceiling plan governs where devices land in the ceiling and is where the architect coordinates them with lights and sprinklers; the mechanical plan and air device schedule govern type, neck size, and CFM. On real jobs the two disagree, and a count that silently picks one set of documents inherits its errors. Count from one, verify against the other, and flag conflicts for an RFI.

More than the device. A typical supply diffuser assembly carries the diffuser itself, a spin-in fitting at the trunk with a balancing damper, a run of flex duct (commonly 4 to 8 ft), straps or supports, sealant, and the labor to set it in the grid and connect it. Returns are usually cheaper because plenum returns need no flex runout. Pricing the bare device and skipping the assembly is one of the quietest ways to underbid the air distribution scope.