The "ELF Application Binary Interface s390x Supplement" is an addition to the generic ELF ABI document. It defines the IBM Z and LinuxONE specific calling and linkage conventions and thereby sets the standard for compiler and linker developers to ensure interoperability between different implementations.
In contrast to the older versions hosted by the Linux Foundation, this document is a major overhaul. Apart from many layout and wording improvements it also covers the vector ABI extensions introduced with IBM z13.
You can access the document here: