HuntingDog, thanks for clearing up that you are talking about Ferrous Coin not High Trash. The behavior of the Fe number is highly affected by which of those two modes you are in, FC or HT. As several have pointed out, in FC, the Fe number is much more constant and is brought up to at or near 12 almost all the time, but in HT, the Fe number bounces around a lot more.
My observations are that in FC, the Fe number is brought to 12 even on what turns out to be iron. So I think the idea behind FC is to let more ferrous targets come in and then the operator has to chose. Sure, more targets sound and look good, but it's only because the machine is bring most of the iron Fe numbers up to 12. FC is no magic bullet in the iron, at least as far as the audio goes.
The TID screen indications are a little more interesting. In High Trash, the curser moves rapidly around, sometimes clearly along a line. Ferrous Coin changes that high speed flickering to forming two blobs about where the ends of the line would be in High Trash, making it a bit easier to see. Some say this is two targets being seen simultaneously, but I've found that if there are two blobs formed, one in the coin area and one in the ferrous area, it's still almost always a ferrous object. The blob in the coin area is just the iron false being tracked.
I'm really not sure what the advantage is to Ferrous Coin. All it seems to do is make the Fe number fluctuate much less, but that doesn't mean the Fe number is any more accurate. Ferrous Coin just takes the Fe number and moves it to 12 and takes it out of the analysis and makes you have to rely much more on just the Conductive number by itself.
By the way, I have two main modes I hunt in: usually mostly in Ferrous Coin, combined audio, and open screen with the Fe tone at 23. I have a second mode that I go too to check targets by hitting the User button, and that is High Trash, 4-tone conductive audio, and open screen. Both modes have a second Disc pattern disc'ing out everything under 23 for when the iron tones get too much. I find Ferrous Coin generally gives me cleaner audio but also makes me dig more iron junk - the iron junk is easier to identify in High Trash where the Fe numbers will bounce more down into the Fe area (17-25, let's say) than the Ferrous Coin mode where no matter what the Fe number is usually 12. I go back and forth between the two modes on all targets I think might be a deep coin so I can try to figure out of there is any advantage to one mode over the other and look for subtle differences.
Last thing: when in High Trash I keep the audio in 4-tone conductive instead of combined because with the bouncing Fe numbers, it's cleans up the audio a lot more. Then I just look at the screen and watch the Fe numbers to tell me if I have deep iron. It's still very tricky to tell the difference between deep iron and a deep coin. About one out of every ten deep (usually small) rusty iron object I dig up turns out to be a deep coin, and for the life of me I can't really tell any difference between a small deep rusty iron object and a coin no matter what I do
. Maybe one out of ten targets turning out to be a deep coin is about as good as it gets. That's not really that bad of a ratio and maybe on a lesser detector, that ratio would be 1 out of 50? There are a lot of guys locally that hunt with the CTX in the same parks I do, so it's possible we've already cleaned out all the good sounding targets and only the iffy ones that sound like iron are left. It's very rare anymore to find a good deep coin out in the open all by itself, even the 7-8" deep coins that aren't being masked are few and far between. Most of my good finds are 7-8" deep *and* are being partially masked.
good to know I am not the only one 
