WCAG 2.0 conformance levels
WCAG 2.0 guidelines are categorized into three levels of conformance in order to meet the needs of different groups and different situations: A (lowest), AA (mid range), and AAA (highest). Conformance at higher levels indicates conformance at lower levels. For example, by conforming to AA, a Web page meets both the A and AA conformance levels. Level A sets a minimum level of accessibility and does not achieve broad accessibility for many situations. For this reason, UC recommends AA conformance for all Web-based information.
The WCAG document does not recommend that Level AAA conformance be required as a general policy for entire sites because it is not possible to satisfy all Level AAA success criteria for some content. See Understanding Levels of Conformance and Conformance Requirements.