From RGDWiki

DIAG is a GameCube diagnostics disc. There are multiple versions of it dating back to 2001 and possibly as far back as 1999, however, the only dumped version is a version from March 2006 (likely the final version before all development was moved to RVL_DIAG), which uses an early version of RVL_DIAG's NMenu and contains many of the same tests, with Wii-specific DOLs removed.

The Gamecube Aging Disc and Gamecube Service Disc are possibly similar to earlier versions of the DIAG disc; as the version of NMenu in the dumped DIAG disc image is an early one and there is evidence that the NMenu program itself was created for Revolution development, it is possible that earlier DIAG discs were very different in structure.

The March 2006 DIAG notably contains a test for the prototype Wii Remote (Dev Tool Version 3).

The dumped DIAG image was obtained from an RVT-H Reader, seemingly copied to it during its factory initialization/testing process and later flagged as deleted.