Mass Ply is made to handle exterior exposure criteria much like plywood and other engineered wood products. It can be exposed during construction periods, but precautions must be taken to minimize exposure, and Mass Ply must be dried out before putting protective covering over it. Expansion and contraction from moisture need to be considered as well.
Please refer to this blog that provides expansion and contraction data and discusses the benefits of Mass Ply in relation to moisture. Mass Ply is not meant to be permanently exposed to the elements. Appropriate siding products should be used in conjunction with the structural panels.