Sheathing is situated in the wet zone of the design and as such needs to be water resistant. Currently the most often used material is OSB3 which is a water resistant OSB. Water resistant plywood can also but used but this is much less common. Both of these solutions require a breather member to be used on top of the sheathing board. Alternatively, latex impregnated wood based fibreboard insulation can be used negating the need for a breather membrane. However, in this case the sheathing board needs to be moved to the inside wall in a 'reverse wall' type of arrangement.
Sheathing has a load bearing function as it needs to resist all lateral loads on the building such as wind loads. Sheathing thickness typically ranges from 9mm to 18mm depending on the anticipated lateral loads.