Code applications: 1-hour rated walls between attached dwelling units, between residential garages and adjacent living space, along stair enclosures, and in commercial corridors per the local building code. Each rated assembly has a specific layered configuration of Type X plus other materials (sheathing, insulation, fire caulking).
Type X is heavier and slightly more expensive than standard 1/2-inch drywall (typically 20-40% more per sheet). Installation takes more labor because the 5/8-inch thickness is harder to score, cut, and handle.
On a takeoff, track Type X area separately from standard board. Different rated assemblies (1-hour, 2-hour) use different layer counts; double-sided 2-hour walls need 2 layers of Type X on each face plus sheathing between, doubling the sheet count.