This page: http://decibel.biography.ms/ mentions both dBv and dBu.
Says in "electrical voltage" (probably with reference to audio
industry), dBu and dBv both mean dB relative to 0.775V -- generally
0.775Vrms, but dB taken as 20 log(V/0.775), without reference to a
particular impedance.

But the same page says dBu or dB(lower-case Greek mu: "micro") as radio
power is dB relative to one microvolt per square meter.

Go figure. It all points out the need to be careful to define your
terms if there's any chance of ambiguity. If you're not careful, your
reader may think dBu refers to Dallas Baptist University, or Deutsche
Billiard Union, or Duluth Business University... though the
capitalization would be wrong for those.

Cheers,
Tom