In the ELF spec, what does BA_OS and KE_OS mean?

Question

In the ELF spec excerpted from the System V ABI, various functions are referenced as exec(BA_OS), as on page 7 of 1. Other functions are referenced with KE_OS. What do these *_OS symbols mean?

Answer 1

It turns out that page 11 of the latest base document describes "components of conforming systems", wherein "BA" means "base system", "KE" means "kernel extension" while "OS" means "operating system service routines" and "LIB" means "general library routines".