In an era of hyper-convergence and custom cloud architectures, some have questioned the longevity of the 19-inch rack. Large players like Facebook (via the Open Compute Project) have experimented with wider racks (21 inches) to improve airflow and accommodate larger motherboards.
The "D" revision specifically refined the technical drawings and tolerance requirements, ensuring that the "unit of measure" was precise enough for mass manufacturing. eia-310-d
A crucial "deep" understanding is what this standard : In an era of hyper-convergence and custom cloud