Package Manager Specification is a set of rules that a package manager compatible with Gentoo package managers (Portage, Pkgcore, Paludis, ...) should follow, it describes how to parse, create and install ebuilds - which are recipes describing how a package source code should be downloaded, compiled and installed.
The PMS document is available here: Package Manager Specification
I have just completed a parser and interpreter for Package Manager Specification (PMS) dependency format.
Shown below are tests ran on real ebuilds (package recipes) available in the Gentoo packages repository.
This is how a PMS dependency specification looks when interpreted as Racket structures:
Source repository: Gentoo Racket / Racket-PMSF · GitLab