There are four dual recording modes -
Off, Dual, Mix, Line
Off - well, the feature is off
Dual - two recordings of the same mic recording at different levels (-6dB to -12dB at 1dB increments)
Mix - one recording of mic and line mixed (mix control 1 to 100)
Line - Mic and line to separate files (level 1 to 100)
In addition the second file in any dual recording mode can be auto gain controlled or limited.
File name for the second file describes the mode used to create it.
All pretty neat at the price point.