Epitaxial growth starts from the surface it lands on. If the c-axis is off by more than the specification allows, the layer that grows is strained differently across the wafer, and the wavelength or the defect density moves with it.
This is why orientation is measured before slicing rather than after polishing: an offcut found after lapping means the ingot was cut on the wrong plane, and the parts are scrap, not rework.
The measurement itself is not exotic — X-ray diffraction against a reference plane, with the reference flat cut in the same setup so the customer can index the wafer the same way we did.
What the tolerance buys is repeatability: every wafer in the batch starts epitaxy from the same plane, so a process tuned once stays tuned.
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