"dBu" simply means "dB unspecified" where the load impedance is not the same as the system impedance.?

0 dBm is 1mW into a 50 ohm load and defines an actual power level in watts.

Any measurement where the load impedance is something other than 50 ohms is automatically in dBu and is useful only for relative measurements such as adjusting for min/max.

dBu is often used to describe signal voltage levels (technically incorrect) by ignoring the impedance and relating the voltage levels that would be present as if the impedance was 50 ohms.? 0dBu is 0.224v at any impedance. 10 dBu is 0.707v at any impedance.