The directive on this page explains explicitly the scope of the page. When used as an edit summary tag it clarifies a page scope problem and notes that it has been corrected.
It should be obvious from the page name what the page is for, but, sometimes, longer detailed explanations are required. Usually links to other pages should be included to direct any unwanted content to the correct page.
Reviewing all pages with such directives regularly helps improve coverage and category relationships.
This is a wiki best practice of the living ontology. This page CC-by-nc-sa by ECG.