Someone said here that there are today lots of more scripts studied than have for now no interchangeability, but that may be still needed for bibliographic references, so that there was already a private registry of private use script codes (PUSC) nearly filling all the PUSC space allocated in ISO 15924 (50 rows, with Code in Qaaa to Qabx).
I understand that this space is limited to 50 numeric codes, but why isn't there space for 10 times more with 4-letter codes? Is it required that all 4-letter PUSC codes have a corresponding numeric code? Could there not exist another separate range of PUSC codes without any associated 3-digits numeric code (the application will choose itself and privately the numeric index to associate for them if it is needed, for example by mapping them starting at 1000, or using negative values) ? If so, why not reserving all codes starting by "Qa" for private use? This would give 676 rows, whose only 50 first ones have a standard 3-digits numeric code, and could allow librarians to create their own private references by giving them full freedom on the last 2 letters of alphabetic codes, so that they could use them mnemonically (for now all they have is "aa" to "az" and ba" to "bx").
