i believe nappa umbria is nappa that has a wax coating, making the leather stiffer and more dull than regular nappa...i agree with 24f that colors in nappa umbria have much more tonal variation than colors in nappa. i'm not sure what part of the process contributes to this.
i believe the nappa umbria is supposed to darken and patina in a really lovely way.
i will throw one more piece of leather info out there but it's not for the faint of heart: bv has also used horse hide. i have a limited edition piece (posted in the bv collections thread) that is made of horse hide. i love horses and was horrified to hear this (the sa felt compelled to ask me several times if i was ok with this to make sure i still wanted it), but i had to have it anyway. i don't know if they used horse hide in any other pieces.