Element: A WCAG requirement for a specific html element or an element with an ARIA role, property or state. For example, an img element must have an alt attribute.
Page: A WCAG requirement that applies to a feature of a web page. For example, a web page must have a main landmark.
Website: A WCAG requrement that applied to pages in a website. For example, consistent navigational links in each web page.
Importance
Element level rules are typically more pass/fail.
Websiteem> and Page level rules are more likely to be manual checks and are difficult to fix if not considered during the design change of a web resource.