Choice between Identical Tai Tham Characters

[Richard Wordingham via Unicode]

What authoritative recommendations or injunctions have been given for
choosing between the encodings <U+1A60 TAI THAM SIGN SAKOT, U+1A37 TAI
THAM LETTER BA> and <U+1A60, U+1A38 TAI THAM LETTER HIGH PA> for the
subscript character known natively as 'hang ba'?  The choice has no
implication as to glyph shape or the pronunciation of the character, and
the only difference in Unicode-associated properties is that the
difference is a primary difference in the DUCET default and CLDR root
collations.

It is quite conceivable that a prescribed choice may be intended to
distinguish homophonous homographs, e.g. ᩈᩣ᩠ᨷ 'bad smell' v. 'curse',
which are usually spelt differently in Northern Thai in the Thai script
and are spelt differently in Thai (สาบ v. สาป).

This subscript consonant is used in all the languages that regularly use
the script.

I can think of some common sense rules such as, "A Pali writing system
should use only one of U+1A37 and U+1A38", but it's not impossible that
even this has been overridden.

The Khmer script has a similar issue with COENG DA and COENG TA, but
between them they represent two different sounds, and TUS recommends
that the encoding be chosen on the basis of the sound.

Richard.