Reposted from apawood.org:
Wood structural panels may be produced in three bond classifications: Exterior, Exposure 1 and Interior. The bond classification relates to adhesive bond, and thus to the structural integrity of the panel. By far, the predominant bond classifications are Exposure 1 and Exterior.
Exterior Plywood Panels are bonded with fully waterproof glue and are suitable for repeated wetting and redrying or long-term exposure to weather or other conditions of similar severity.
Exposure 1 Plywood Panels are suitable for uses not permanently exposed to weather. Panels classified as Exposure 1 are intended to resist the effects of moisture due to construction delays, or other conditions of similar severity. These panels are manufactured with the same fully waterproof glues as used in Exterior plywood; however, because of other compositional factors that may affect bond performance, only Exterior panels should be used for long-term exposure to weather. Exposure 1 panels may, however, be used where exposure to the outdoors is on the underside only, such as at roof overhangs.
APA Rated Sheathing Exposure 1 plywood, commonly called CDX, is oftentimes confused with an Exterior plywood panel and misused in applications. It is important to understand the difference between these classifications in order to select the best panel for your project.
On the APA trademarks shown here, the bond classifications are described as Exposure 1 (left) and Exterior (right).