**From Marcel’s link:
Exterior Gypsum Sheathing is a water-resistant product designed for attachment to exterior side-wall framing as an underlayment for various exterior siding materials such as wood, metal or vinyl siding, masonry veneer, stucco, shingles, etc. The panel is manufactured with a wax-treated, water-resistant core faced with water-repellent paper on both face and back surfaces and long edges.
Exterior Gypsum Sheathing is available in a 1/2" thick, 2’ wide with a tongue and groove edge or, 1/2" & 5/8" thick, 4’ wide square edge product. 5/8" Exterior Gypsum Sheathing has a Type X core, for use in fire rated assemblies.
Gypsum wallboard manufactured by American Gypsum contains no asbestos.
Exterior Gypsum Sheathing is recommended for use in residential or commercial buildings, and provides fire resistance and additional structural strength when used under exterior finishes such as wood, metal or vinyl siding, masonry veneer, stucco, shingles, etc. Exterior Gypsum Sheathing is designed for direct attachment to wood framing with galvanized nails **or ****staples** and screws, or metal framing by the use of screws. **The joints are not intended for finishing with drywall joint compound and tape.**