What does AA vs AAA mean for WCAG Contrast?

Using Firefox and its Web Developer tools, I am seeing a score of 5.5 rated AAA, with another of almost 7 rated AA.

I've searched for WCAG score vs rating and a couple related searches but found no instance of even an attempt at an explanation. Can anyone shed some light on what I need to know to better predict and understand the number-scale and rating-scale relationship?

Based on your screenshots, it looks like what you're evaluating is the contrast minimum of two colours.

So the number you're seeing is not a "score", but the contrast ratio between the two colours you've selected.

A, AA, and AAA are the 3 levels of conformance for WCAG. Very quickly, most policies and laws require certain categories of organizations' websites in their jurisdiction to meet the specified level of conformance. So a website would need to pass all WCAG criteria at AA, for example.

The contrast ratio for

• AA: at least 4.5:1
• AAA: at least 7:1

So anything between 4.5 and 7 is AA, and 7+ is AAA. This is reflected in your second screenshot.

For large text though, there is a separate contrast ratio:

• AA: at least 3:1
• AAA: at least 4.5:1

Your first screenshot shows that it's evaluating the contrast ratio for "large text", so in this case anything between 3 and 4.5 is AA, and 4.5+ is AAA.