The Authoring Tool Accessibility Guidelines (ATAG) are a set of requirements for making authoring tools that are accessible and can produce accessible content. ATAG 1.0 was released in 2000 and it was replaced by the current, ATAG 2.0, which was released in 2015.
Similar to WCAG (Web Content Accessibility Guidelines), ATAG is published by the World Wide Web Consortium (W3C). WCAG is intended for content authors, and ATAG is intended for the developers of authoring tools.
ATAG is used to check if the author's experience is accessible and if what they create can be made accessible to the end users. When an authoring tool meets ATAG, it means it can be used by people with disabilities to create content that is accessible to people with disabilities.
No, ATAG is not currently required by law. When you are choosing what authoring tool to use, I recommend selecting a tool that conforms to ATAG (level AA). That way you will help all your users.