It was the first pocket PC running Windows Mobile 2003 Second Edition. There were 3 models, 2 with a 312MHz processor and one with a 624MHz Processor. That 624MHz processor was the fastest in a pocket PC at the time. The lowest model had no Bluetooth or WIFI and only 32MB ram, but the other 2 had 64MB ram had Bluetooth and WIFI.
The Dell Axim series started in 2002 with the Axim X5, and finished with the Axim X51 until discontinued in 2007. Models were X3, X3i, X30, X5, X50, X50v, X51, X51v most had high end and low end variants.