Wafer flats convention, based on Image:Wafer flats convention.PNG
Conventional meaning of flats in semiconductor wafers.  Red denotes material that has been removed.
Wafer orientation is the orientation of the crystallographic plane in which the crystal grew. Wafer type indicated the type of doping.
Primary flat - indicates crystallographic planes of high symmetry (usually the {110} face).
Secondary flat - it's position relative to Primary flat or it's absence is indicating the doping type and the orientation of wafer.

Absence - p {111}
90° - p {100}
180° - n {100}
45° - n {111}
Wafers under 200mm generally have flats indicating crystallographic planes of high symmetry (usually the {110} face) and, in old-fashined wafers (those below about 100mm diameter), the wafer's orientation and doping type.  Modern wafers use a notch to convey this information, in order to waste less material.  Orientation is important for wafer cleavage, and can affect other structural and electronic properties as well.
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Twisp