What is the output level when [0] on the master level indicator is lit?

If the MASTER OUT [ATT.] is set to 0 dB, the MASTER output (XLR) will be +4 dBu when the master level indicator 0 is lit, and the MASTER2 output (RCA) will be 0 dBu.

Note that even if you adjust the [ATT.] of the MASTER OUT, the master level indicator doesn't change.