Use of tag characters in a private encoding - is it valid please?

[Asmus Freytag]

On 5/7/2024 11:16 AM, Erik Carvalhal Miller via Unicode wrote:
> “The current conformant use of the undeprecated 96 tag characters is 
> specified in Unicode Technical Standard #51, ‘Unicode Emoji.’  See 
> ED-14a. emoji tag sequence (ETS) and Annex C, Valid Emoji Tag 
> Sequences in that specification.”  No, the Standard itself (book or 
> collection) does not define what emoji tag sequences are or which ones 
> are valid; but that same Standard points to UTS #51 as the definitive 
> specification of ETSs for “conformant use” of tag characters. 

Contrary to your reading, the correct interpretation of that passage is 
one that considers the tag characters as quasi reserved for use with a 
specific external protocol. The word "current" allows Unicode to later 
designate other protocols, if desired.

It very clearly does not contemplate any other uses of tag characters, 
so no, their use with PUA characters (or any other characters) is not 
conformant in the same way as assigning a PUA code point a private 
character.

In order to use the tag characters conformantly, you must claim 
conformance to both TUS and UTS#51.

A./
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