Semantic markup is used as HTML markup tools to reinforce the Semantics, or meaning, of the information in webpages and web application rather tah just simple presentations or looks.
Section 508 Accessibility
Amendement to the United States Workforce Rehabilitation Act of 1973, stating that it is a federal law mandating that all electronic and information technology developed, procured, maintained, or used by the federal government to be accessible to all with disabilities.
SEO(Search Engine Optimization)
Process of affecting the visibility of a website or a web page in a web search engine's unpaid result. In marketing, this methodology allows an increase amount of visitors by giving them a high rank on the results page of a search engine.
Overall, nowadays, all three are used to help those in need access sites easier.