Section 501 of the Rehabilitation Act obligates federal executive branch agencies to affirmatively hire, promote, and accommodate employees with disabilities, applying employment discrimination standards equivalent to Title I of the ADA. It is frequently confused with Section 508, but the two provisions cover different ground.
Section 501 is about federal employment practices; Section 508 is about the accessibility of the electronic and information technology those agencies develop, procure, and use. Consultants working with federal agencies typically encounter Section 508 far more often, since it is the provision that governs website and software accessibility specifically.