udma 100 and ATA 100 mean the same thing.. connotatively anyways..
ATA - (AT attachment) and was a developed as a standard long before dma or udma transfer modes were developed for ATA/ATAPI devices.
UDMA - ultra direct memory addressing
The confusion probaby comes from some drive manufacturers using the label ATA (which is really supposed to be a standard that defines features) and appending the 33/66/100 for those udma transfer modes, which leads to other manufacturers of various components doing the same. Yet there really is no such thing as a ATA 100 transfer mode, but they still do it. I don't know why this got started beyond marketing reasons..(It might of been a Quantum thing.. I'm not sure) it's real easy to do though since ATA/ATAPI features that define the newer standards are basically limited to the increased bandwidth additions. For whatever reason and for all intended purposes, ATA33/66/100 will mean the same as UDMA33/66/100.