The digital volume control also limits the output of the signal to the point that you can't overdrive a high wattage amp. With no boost (0db) I can max the digital volume and not be too loud. That saves inadvertent mistakes .
I love this thing's versatility. A digital/analog Swiss army knife.
Maybe I misunderstand you.
It's my impression that the different Line Out Trim settings described on page 23 in the user manual (April 2012 v. 4) are analog, not digital. The higher levels are probably (only) intended for use with professional equipment.
The digital volume control for the line out is a separate thing discussed (briefly) on page 30 of the user manual.