While reading about ID_Start and ID_Continue definitions, I found this: https://unicode.org/reports/tr31/#D1. It says that ID_Start includes Other_ID_Start and ID_Continue includes Other_ID_Continue. I'm unable to find the definitions of these other. The document I mentioned says that they're defined by UAX44. So for example, I tried to consult Unicode 15 version of UAX44: https://www.unicode.org/reports/tr44/tr44-30.html The table 9 (Property Table) only says:
Other_ID_Start Used to maintain backward compatibility of ID_Start.
Other than that, there is no additional information. What am I missing?
Other_ID_Start and Other_ID_Continue, like most binary character properties, are defined in the PropList.txt data file in the Unicode Character Database.
In particular, Other_ID_Start includes characters that used to be included in ID_Start automatically due to some other property they possessed, but now need to be specified manually because said property value has since changed. For example, U+212E ESTIMATED SYMBOL was originally classified as a letter and all letters are ID_Start by default, but later it was reclassified as a symbol and thus would have been excluded if it weren’t for the backwards compatibility requirement.