When making software procurement decisions, educational entities need to look critically at which products in a given software category will best serve the needs of all students, without excluding some on the basis of disability. To determine whether a software application is accessible, the application should be evaluated according to a set of guidelines or standards that defines software accessibility.
The only legal standard is Section 1194.21 ("Software applications and operating systems") of Electronic and Information Technology Accessibility Standards . These standards were developed by the Access Board as required by Section 508 of the Rehabilitation Act Amendments of 1998. There are twelve standards, and the Access Board provides a detailed explanation of each in its Guide to the Standards .
Additional guidelines and resources are provided in the AccessIT Knowledge Base article How can I tell whether a software application is accessible? 
-  Electronic and Information Technology Accessibility Standards
-  Guide to the Standards
-  How can I tell whether a software application is accessible?