Hello, I have a Quattro, which should be the same. To me it seems that either the TID meter is based on "ferrous" or is a combination of both. Let me explain why I think that.
When using "ferrous" audio, the tones correspond quite well to the numbers I see on the screen. Low tone for negative numbers and the higher number, the higher tone it is.
However, in "conductive" sounds, iron (minus numbers) sounds a high tone as iron is a relatively good conductor. Cu-Ni, nickel coins, aluminium give a middle tone in both modes, gold as well, and register
somewhere around 10-20. Copper and aluminium bronze coins give a high tone as well, but the meter reads above 30.
Therefore I hunt in "conductive" audio and I think the numbers show me information about ferrous content. I have some 20 hours only with the Quattro, but the things I find seem to support my theory. Anyway, to me the "conductive" mode seems to give more information about the target.