Semantic markup in HTML is used as a way to reinforce the meaning of information, rather than just defining its look.

Section 508-Accessibility

This requires all federal agencies electronic information be accessible by people with disabilities.

SEO

This is building websites that can be easily identified by search engines. This achieved through guidelines for proper HTML

How are these relatied?

SEO and 508 both rely on proper semantic markup to ensure compatability with eachother.