This Corrigendum addresses a defect in ITU-T Rec. X.691 | ISO/IEC 8825-2 arising from a change in wording between the 1997 version and the 2002 version which is considered ambiguous.
The ambiguity relates to the encoding of a type that is not in the extension root of a character string type. The clarification is to ensure that it is still encoded (after the setting of the extension bit) using the effective permitted alphabet (which itself can never be extensible), but without an effective size constraint (which can be extensible).
This is arguably implied by the current text, but some interpretations assumed that the (non-extensible) effective permitted alphabet constraint was (unnecessarily) discarded when encoding values outside the root of the extensible effective size constraint. The clarification is to state that this is not the case.