PMS provides the specification of ebuild format, as well as general guidelines for implementing package managers. All ebuilds in the Gentoo repository are required to conform to the PMS. Tree policies may enforce additional restrictions upon the format discussed in PMS.
PMS is maintained by the PMS project. All major changes are done in subsequent EAPIs that are approved by the Council. The project's wiki page discusses how PMS can be changed via future EAPI process.
GLEPs provide the highest level policies applicable to Gentoo. Final or active GLEPs apply to all developers. Tree policies may impose additional restrictions on GLEPs but may not override them.
The process for creating and updating GLEPs is documented in GLEP 1. In general, all GLEP updates go through mailing list review and need to be approved by the Council.
Devmanual is the basic guide for ebuild developers. Besides policies, it contains many general recommendations and detailed instructions. Developer Manual does not specify policies itself, and needs to comply with policies defined in this document.
Technically, devmanual can be changed by any developer. However, it is recommended that all changes are reviewed by the devmanual project.
POSIX is the basic standard for operating systems. However, its rules apply to the software packaged in Gentoo rather than the distribution itself. Nevertheless, when no more specific policy applies, following POSIX is recommended.
FHS specifies the suggested filesystem layout for Linux systems. Gentoo follows FHS only partially. Whenever Gentoo policies and FHS disagree, Gentoo policies should be followed.