A good read that very clearly explains the connection between WCAG 2.0 and PDF/UA (which is really a set of suggestions to complement another set of suggestions because we live in the wild west).
Strongly Encouraged Fixes (SEFs): issues that are violations of WCAG 2.0 and will be elevated to Required Fixes once the 508 Refresh goes into effect; they should be proactively fixed now.
Encouraged Fixes (EFs): issues that should be corrected in order to meet best-practice accessibility standards but are not required for conformance with Section 508 of the Rehabilitation Act or HHS accessibility guidelines.
ASPA-DCD classifies accessibility errors into three categories as shown below:
Required Fixes (RFs): violations that must be corrected and have direct mappings to either Section 508 or HHS accessibility guidelines.