One bar is 2" or less, two bars is 2 to 4", three bars is 4 to 6" or more, but that is for coin sized targets and the F2 is not always accurate on these.
It definitely is not correct on tiny objects which will show deeper than they really are, or large targets like cans which will show much shallower.
I don't use those bars to gauge dept much, the pinpoint numbers on coin sized objects might be off an inch or so but they are more accurate than those bars.
What I do use those bars for is to determine trash.
When I have the target centered under the coil I swing the coil in a few short side to side movements over that target.
If the bars jump up and down there is a good chance the target is irregular shaped trash because even though some trash will be stable doing this, most good targets usually will be too and won't cause those bars to jump much if at all.