What is the significance of primary and secondary flats in wafers?

Question

I am confused about how the orientation of primary and secondary flats represents the crystal orientation of the semiconductor. I looked at this site mentioned in this answer:

The primary flat has a specific crystal orientation relative to the wafer surface; major flat.

Is the "specific crystal orientation" the same for all wafers? What is "major flat"?

Secondary flat Indicates the crystal orientation and doping of the wafer.

How does it do that? For example what does a 45° angle between primary and secondary flats represent? Is there an actual physical correspondence between flat angles and crystal orientation? Or is it just by convention?

Answer 1

There is no physical relationship between flats and the orientation or doping of the wafer. It is purely convention.

The primary (large) flat indicates the orientation of the crystals, and, if present, the secondary (smaller) flat indicates the doping.

The numbers in braces are the Miller Index of the crystal orientation.

This convention is only used on smaller (< 200mm) wafers. Larger ones use a notch to indicate the orientation and do not include doping information.