Site-Policy is the way the main site is run and managed - processes, procedures, and routines that should be followed. Use this tag when you believe site-policy has not been applied correctly to a main-site question, or when you believe existing site policies should be modified, or new policies added.
Site policy should not be confused with scope.
- scope is about what sort of questions are on-topic
- site-policy is about rules, processes, and routines that apply to questions whether they are on-topic or not.
Note that Stack Exchange rules always apply on all sites. The site-policy rules simply add to them.