To avoid confusion,the capital "V" was then made the 1-volt zero reference standard by the International Electrotechnical Commission (IEC),while the NAB agreed to use a small "u" to denote the voltage value that might be obtained when the customary 600 ohm load is used to measure the dBm (although the load itself must be minimal).
The "u" in "dBu" thus stands for "unloaded," a term engineers use to describe an output which works into no load (an open circuit) or an insignificant load (such as the typical high impedance inputs of modern audio equipment).