They are choices of Y axis units for the SPL & Phase graph. From the help:
The left hand Y axis can be set to show dB SPL, dBFS, dBr, dB V/V, dBu, dBV, dBW, volts, watts or, for impedance traces, ohms. The dBr and dB V/V values are effectively a transfer function view, showing either the relative input to output dBFS levels (dBr) or voltage levels (dB V/V). Values for dBW and watts are derived from the measured voltages using the reference impedance value in the RTA Appearance settings.
dBu is a logarithmic unit of voltage,# often used to describe voltage levels in professional audio applications. It shows voltages in dB relative to 774.6 mV (which is the voltage that produces 1 mW into a load of 600 ohms)
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