Not sure exactly what you want to know, HighVDI. The simple answer is, they are "Ferrous" and "Conductive" numbers (which I'm sure you already know), though those are really just names given to them by the engineers. What they are "exactly," which is what you asked, gets very technical, very quickly, and involves concepts such as the "reactive component" of a receive signal, a target's "time constant," etc. If you want more info on these things, I can give you a good link.
But I'm guessing what you are really asking is, how do you use them to your advantage, right? Basically, just think of it as a "more detailed" way of identifying a target -- instead of say 0-99 segments of ID (100 different numbers), you instead have 1,750 different two-digit combinations to identify targets with, on an E-Trac or a CTX (which is why there are 1,750 "pixels" on your smartfind screen). The way the receive signals are processed in FBS allows a more more accurate "binning" of targets into proper categories, particularly in soil that is less than "mild," and particularly on deeper targets. Iron content in the ground obviously causes "ID inaccuracy" problems, and by analyzing the FE and CO properties of targets, Minelab's FBS algorithms more effectively identify targets than traditional single-frequency VLF machines.
The bottom line is, as I'm sure you know, most "good" targets, in average soil, that aren't TOO deep, will generally have a FE number near "12," while nails and such will usually have FE numbers in the low to mid 30s.
Not sure if I've answered your question at all, or not...
Steve
