Atmel has several series of XMEGA chips. How does the numbering scheme work?
Nice Atmel brochure. Good general overview and all members of family listed on bottom of page 11.
Whence cometh this ...
From the current parts list, I gather the first number
nnn is the amount of flash in kilobytes. The letter
L is one of
D, the cheaper
D parts have no DMA or DAC. The number
m is 1, 3, 4 for 100, 64, 44 pin parts, and a
U suffix signifies a part with USB.