Policies found on this wiki are divided between local (Meta) which apply to how Meta itself runs, and global which are used to define, govern and resolve things for the entire platform.
In either case, the pages are often protected from edits and even if you can edit them, they should only be corrected for grammar or spelling errors, translation tasks and related small tweaks. Anything to change the wording or meaning of the policies requires the backing of a Request for Comment, the formal process for the community to decide changes to policy. The process can be used for both Meta and global policy.
Local wikis may develop policy however they wish as long as it does not contradict global policy, particularly Content Policy. However, if no local policies or clear conventions can be found, global operatives will likely review things based on their understanding of how Meta works as a default template.
Supplementary information that is found in Category:Documentation includes policies, but also information which defines convention and offers supplementary context to policy. Text from policy and the collective discretion of authorized users ultimately overrides text which is found in non-policy documentation.