If you have a RISC-V machine with an operating system and a C compiler then it will already include all the extensions from RVA20 (older boards such as VisionFi

If you have a RISC-V machine with an operating system and a C compiler then it will already include all the extensions from RVA20 (older boards such as VisionFive 2, Milk-V Pioneer, LicheePi 4A, Milk-V Duo, HiFive Premier), or RVA22 (the very newest boards such as LicheePi 3A, Milk-V Jupiter, DC-Roma II, BananaPi BPI-F3) ... and in 2 or 3 years boards meeting this RVA23 spec. The only things that have mix-and-match extensions are embedded devices where you choose exactly what chip you buy (or build), you know what extensions it has, and you compile for that set of extensions. There is no confusion because everything is under the control of one company. Machines intended to run software distributed in binary form follow the RVA specs which are few in number, only added to every few years, and each one includes everything in all the previous ones. There is again no confusion -- or at least no more than in any other ecosystem that is not permanently frozen. RVA23 is pretty much equivalent to ARMv9 or the latest x86_64 spec with a variable length version of AVX-512.