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.