The UMA Acceleration Architecture (UXA) is a 2D acceleration method designed by Packard that uses the EXA API but with all of the internal code using their Graphics Execution Manager for memory management.
Introducing a new acceleration architecture that is dependent upon the GEM memory manager has led to some confusion and upset among X.Org developers, but Keith Packard has pledged that UXA will be discontinued. Once he decides where to separate the acceleration architecture for pixmap management and the acceleration architecture, Keith will stop developing UXA and will merge these changes into EXA.
The new EXA will then be split into pixmap management and acceleration architecture components, to allow each individual driver to decide whether to use both sides of EXA or not. Keith hopes all of this work merging UXA back to EXA will be done in time for the X Server 1.6 release.