For flush riveting, the PDF includes the exact radius and depth of the 100° countersink. A mismatch between the rivet head and the countersink angle creates a stress riser. The NAS523 PDF includes go/no-go gauge dimensions.
Before NAS523, different manufacturers might have used different codes to identify similar rivets. This created chaos in the supply chain and on the maintenance hangar floor. NAS523 standardized this into a logical format that tells you everything you need to know about a fastener just by looking at its part number. nas523 rivet code pdf