Crystal orientation determines the characteristics of sapphire windows. The choice of crystal orientations depends on many factors e.g. birefringence, thermal expansion, lattice constant and other crystal orientation dependent physical properties.
For example, C-plane sapphire is slightly birefringent. Sometimes specifized as ‘zero degree’, or ‘c-cut’. In a window, the optical axis of sapphire should be perpendicular to the plane of the window. Most sapphire windows require low birefringence, so C-plane is typical material for sapphire windows.
If unspecified, Random should be the default crystal choice for sapphire windows. The window is fabricated without concern about crystalline orientation, called ‘random’ cut. But need to note that this is nearly 60° to the optic axis as this is the ‘softest’ direction for the saw.