mATX = uATX
microATX/SFX/uATX Form Factor - Computer manufacturers continue to want to shrink the size of their systems, and Intel has been happy to oblige, with another variant on the basic ATX theme called "microATX". As the name implies, microATX is not only smaller still than Mini ATX, it is actually smaller than even NLX. microATX is intended for the very low end market that Intel has decided to target with force.
Technically, it could be said that microATX is not even really a case form factor, because microATX is in fact a motherboard standard. However, with the smaller microATX motherboards, manufacturers are creating smaller cases to fit. To complement microATX, Intel created a new, low-output power supply design called SFX. SFX power supplies are designed to work with not only microATX systems, but also NLX and regular ATX systems, and this is why these power supplies were not called microATX. SFX and microATX are sometimes used interchangeably, or together, in describing this general physical format. The term uATX may also be used to refer to microATX form factor.
http://www.directron.com/case.html