Section 508
A section of the US Rehabilitation Act requiring federal agencies' information technology to be accessible — typically benchmarked against WCAG 2.0 AA or higher.
Section 508 applies to federal agencies and federally-funded programs. Procurement of any IT (including captioning platforms) for federal use is subject to Section 508 conformance requirements.
The 2017 Section 508 refresh aligned the standard with WCAG 2.0 AA. For captioning vendors selling into federal use, Section 508 conformance documentation (often called a VPAT — Voluntary Product Accessibility Template) is typically required as part of the procurement process.
For private-sector event organizers, Section 508 doesn't apply directly but is often referenced in contracts where federal funding flows through (universities accepting federal grants, contractors performing federal work).