Package Manager Specification

The Package Manager Specification (PMS) is ebuild file format, the ebuild repository format (of which the Gentoo ebuild repository is the main incarnation), as well as behavior of the package managers interacting with these ebuilds is properly agreed upon and documented.]]

By defining the ebuild format and package managers behavior, it is possible for software developers to create other package managers next to Portage that are fully compatible with the ebuilds and can be used on Gentoo (and related) distributions.

For more information, see Gentoo's Package Manager Specification project.

Notice the initialism PMS in this sense is a Gentoo developers jargon - somehow confusingly, in Linux in general it often means "Package management system", like portage, pacman for ArchLinux, or apt for Debian.