Hard to find explanation on conformance level (i.e. A AA AAA) on WAI

Hello All:

Working on my web accessibility course, I've had a hard time finding a definition or explanation of levels of conformance (i.e. A, AA, AAA) on the WAI site.

When I follow the link below - three levels: A, AA, and AAA<https://www.w3.org/WAI/WCAG21/Understanding/conformance#levels> - on the WCAG Overview page, you arrive at https://www.w3.org/WAI/WCAG21/Understanding/conformance#levels, which, although it mentions levels of conformance, there is no explanation of conformance levels (except for what is below) and not even a mention of AA or AAA and what it means.
Understanding Conformance<https://www.w3.org/WAI/WCAG21/Understanding/conformance#levels>
Most standards only have one level of conformance. In order to accommodate different situations that may require or allow greater levels of accessibility than others, WCAG 2.0 has three levels of conformance, and therefore, three levels of Success Criteria.
www.w3.org
The information can be found at https://www.w3.org/TR/UNDERSTANDING-WCAG20/conformance.html#uc-conformance-requirements-head but I only came across this second page by googling it. I'm not sure how these 2 pages relate - perhaps this one is an antecedent of the WCAG21 version - but there seems to be some crucial information missing from https://www.w3.org/WAI/WCAG21/Understanding/conformance#levels that is on https://www.w3.org/TR/UNDERSTANDING-WCAG20/conformance.html.
Understanding Conformance<https://www.w3.org/WAI/WCAG21/Understanding/conformance#levels>
Most standards only have one level of conformance. In order to accommodate different situations that may require or allow greater levels of accessibility than others, WCAG 2.0 has three levels of conformance, and therefore, three levels of Success Criteria.
www.w3.org

Just thought this would be of interest.

-Howard