ISO 646 alternate representation

[Doug Gwyn]

In article <keld.671027491 at dkuugin> keld at login.dkuug.dk (Keld J|rn Simonsen) writes:
>ISO 9899:1990 supports ISO DIS 646 IRV, not the invariant part of 646.

I hope you'll explain this, as I thought that ISO 9899:1990 supported
use of that portion of ISO 646 that was common to all national variants.
If that is not "the invariant part", then what is?