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From WCAG WG
WCAG definition of supplemental content
additional content that illustrates or clarifies the primary content
Example 1: An audio version of a Web page.
Example 2: An illustration of a complex process.
Example 3: A paragraph summarizing the major outcomes and recommendations made in a research study.
Additional guidance when applying to Electronic Documents and Software Aspects of Products
This applies directly as written, and as described in WCAG 2.0 glossary replacing "Web Content" with "Content".