The origin of the Decibel Unloaded (dBu) lies with the American telephone company Bell. This company had determined a reference power of 1 mW at a resistance of 600 ohms for the level determination. The impedance of 600 ohms comes from telephone technology. This level corresponds to 0 dBm ( decibel milliwatt).
If we relate the reference power to a voltage value (U), then the power (P) is calculated from 'U^2/R': the square of the voltage divided by the resistance of 600 ohms. This results in a voltage of 0.7746 V. This voltage level was the reference value for 0 dBv. However, since in Europe the dBV (capital "V") is used as the reference value for 1.0 V, the dBv was renamed dBu to avoid confusion.
The reference value can be used to calculate the absolute voltage value. Here, all levels that are smaller than 0 dBu have a negative sign, those that are larger than 0 dBu have a positive sign.
You can convert any dBu value to a dBV value and vice versa: 1 dBV corresponds to +2.2 dBu, 1 dBu corresponds to 0.869 V, 4 dBu, the nominal level used in professional audio, corresponds to 1.228 V, -10 dBV, the level used in semi-professional audio, corresponds to -7.8 dBu.