dB is basically a proportional value, which is a unit of expression of a multiple. That is, the relative difference between the test data and the reference standard.
dB = 10 log(P1/P2)= 20 log(V1 / V2)
V1 is the test data and V2 is the reference standard. For example, the V1 data is twice that of V2, which is 6 dB. (P stands for power and V stands for voltage).
dBV is a 0dB reference standard of 1V, and dBuV is a 0dB reference standard of 1uV. (Generally speaking, the signal strength dB or dBu is actually dBuV)