While UI Automation is trumpeted as "royalty-free", IAccessible2 claims to be an "open standard".
Whereas UI Automation marks a radical break from Microsoft Active Accessibility (MSAA) to create a more flexible accessibility API, IAccessible2 fills in perceived omissions in MSAA to match the Java Accessibility API and Assistive Technology Service Provider Interface (AT-SPI). By extending the MSAA interface, rather than replacing it, IAccessible2 allows "application developers to leverage their investment in MSAA while also providing an Assistive Technology (AT) access to rich document applications.
Project Missouri was started when the State of Massachusetts, in the process of adopting the OpenDocument format for its public records, required that the format be made accessible. Increasing the accessibility of dynamic web applications, for example by exposing custom controls for use with assistive technology and by filtering streams of new information by type and importance, became a second major focus for the project.
Currently, support for IAccessible is in development for applications such as the Mozilla Application Suite and NonVisual Desktop Access. Although IAccessible2 was introduced as a Windows accessibility API, Trolltech are treating IAccessible2 as a potential alternative to AT-SPI on Linux too, and planning to add preliminary support to QT 4.3.