The problem with iron is that though it gives the magnetic response (reactive) and the domains align temporarily to the EM field if the iron piece is flat, long and thin, or oddly shaped with bends and sharp points it also spins eddies that decay just like a non-ferrous target (resistive). So, it is often a bit of both - which will drive anyone hunting in iron nuts with a cursor bouncing about as reactive and resistive signals dance about with each slight coil movement. But because it has a response that is not common with non-ferrous targets it at least reveals itself well, though you have to decide from the info at hand if it might actually be a mixed target under the coil (non-ferrous next to ferrous). Sometimes you dig and 14" down there is that bloody nail or wire or iron bolt - and you got fooled. One thing about iron - since it is magnetic plus eddy producing (often) it produces a strong signal - often very strong. When the E-trac shows 12" depth and the target sounds loud and shallow (with the TID jumping) it is likely to be iron acting like a shallower non-ferrous target. With the E-trac Vol Gain above 28, all targets sound equally like surface targets. This is going to make audio differentiation between deep and shallow targets impossible. So, in iron it would be advisable to drop this down so you can get the true audio strength rather than an equalized audio. If it sounds strong but the depth is 12" (assuming it is correct) then it may be iron (a nail).
I would say, if you've dug iron and you know it is littered about a site, then use that to your advantage and simply assume that a target that repeats in a similar manner as the last 3 dug iron targets (junk) are also going to give you junk. I'm prone to doubt myself after passing the same target response and bite the bullet and dig - just to find that nail at 8" again. If a site is new, dig those iffy targets to get a feel for what's likely to be repeatedly displayed. Time is limited and you can't dig junk targets if they represent over and over again in a similar fashion. I know iron relics are going to be similar in nature to iron junk - so it may be that all you can do is try and weed out a few of the repetitive markers. I don't dig relics so iron is mostly a nuisance.
The reason I said to turn down Sensitivity is to reduce the eddy current strength, which is weaker than the magnetic response. It will help to reveal iron as iron - but it will cost you depth. Sensitivity setting has the single largest effect on depth of recovery and sensitivity to smaller, less conductive targets. They can be lost - especially gold rings which lack conductivity and if thin are likely not to give a large signal response anyway. But if nails are creating havoc it should calm down the TID.
The reason Auto is often preferred is because it suppresses for you the erratic channels - picking the best channels at the highest sensitivity possible (not going above your Auto setting of course). You know that Manual runs all three channels (low, med, high) at whatever level you set it - and you can bet in iron you will get falsing constantly if set above 24. Use Auto +3 and if that is too noisy/jumpy drop the setting down until the iron is manageable. Manual will go deeper but it is useless if you are getting erratic operation (someone said high beam in fog). You could likewise start in Auto - check the suggested Sens (right bar), and then switch to Manual and lower to approximately the suggested level you had in Auto. Then raise or lower as necessary until you limit the falsing. You might get better depth. Generally, use Manual in low iron settings and Auto in trashy or mineralized soils.
Use High Trash Setting - if you have Disc'd areas (blacked squares) it will keep the cursor off them and allow only the un-Disc'd areas to respond. If you detect with All Metal (like QuickMask without Disc then it wouldn't make any difference at all). I leave my E-trac on High Trash Setting all the time. One less thing to think about.
If you go with Recovery Deep "On" you need to slow down and give the detector time to process what is illuminated under the coil. Deep On will boost the audio on weak signals (unlike Vol Gain which which affects both weak and strong signals). If you're looking for deep targets that are fringe (weak), then put Recovery Deep "On". The response tends to be 'smoothed' due to the internal processing, that is less of a quick blip and more of a smoother bump with more audio. That quick "blip" may be something you need to explore and with Deep "On" you will likely hear it without passing it by. But, Deep "On" can smooth several targets together which is confusing to the TID - tending to jump about as the coil position changes. For relic hunting Deep "On" is generally recommended.
Recovery Fast can help compensate for Deep "On", it will chop the signal so that it doesn't bleed into adjacent signals in trashy sites. In other words, it resets the audio and TID so you can differentiate targets in close proximity. Some people don't like it and would rather just slow their sweeps to help isolate adjacent targets (and use a small coil to aid separation). I haven't found it helpful or to hurt - you can experiment with it.
Unless you're on a Florida beach with Tom or open fields, use Ground Difficult. If the Suggested Auto Sens (right bar) is below about 22 (out of 30) then use Difficult (the lower the bar reading the worse the soil). Most soils are Difficult so I generally just leave this one alone.
I use 2-tones most of the time - it helps make audio responses rather simple. Many people love Multi-tone. Whatever floats your boat for you is what you should be using.
Generally, in high ferrous trash areas where the E-trac is nulling, use TTF or MTF. You get a low tone on ferrous readings above 17 (lower on the screen or 18-35Fe). You can use an "open" screen (no Disc) and just listen for that high tone audio. Pretty simple and probably best for relic hunting. You can Disc out some of the bothersome High Fe numbers (that is, Disc out 26-30Fe across the bottom, unless that is where your targets is expected to be. For me, I have no use for these high Fe numbers unless the TID is jumping about from low-high Fe numbers - that is not usually just isolated iron and needs investigating). I'm willing to lose war pennies (well, not really - I know where they fall and would probably dig to be sure).
Generally, in high non-ferrous trash areas use TTC or MTC. In TTC you get the low grunt when Conduct is below 25 (1-24Co). Problem for me is that too many good targets are mixed in that area. It doesn't help to get a low disinterested grunt from the machine because I'm actually intensely interested in this area. If all you want is high silver coins, copper pennies, quarters, and silver jewelry then this setting is fine since they fall mostly above 24Co and will give that high silver tone. But if you want gold, nickels, and less conductive jewelry that falls in that grunt tone area, you don't want TTC.
Threshold Level and Volume Limit are two setting that make little difference. If you like to hear the Threshold louder - turn it up. You do want to hear the detector null (audio dips away to nothing briefly around iron - investigate those areas carefully, there may be something good hidden in there).
Threshold Pitch is useful, but not critical. Set Pitch to low (1-5) and the spatial range between the low tone and high tone is increased. That is, you get a really low grunt and a really high high tone. I use about 8-10 - but use whatever you like (really doesn't matter in the big scheme of things).
Run Noise Cancel after you've made all the setting adjustments to the detector. Some say keeping the coil motionless on the ground is the cat's meow. Others say to hold it stationary off the ground. I say it is unimportant.
I use Normal Pinpointing but if it is on Sizing then I'll use that too. In pinpoint Normal, the response narrows as you wiggle around the target helping to isolate it. Sizing is like VCO tones - the pitch varies by the strength of the response and the volume is loudest when centered over the target. In either case, just make sure you are "isolating" the correct target, you don't want the stronger ferrous junk adjacent to your desired target to be pinpointed. Select pinpoint when just off-center of the target and you'll get a very truncated audio that will locate the target center in seconds. Careful, miss a bit and press pinpoint over the target's strongest signal and its audio will be gone.
The E-trac is very simple to use and setup. It's a bit heavy but it is manageable for me (I hunt an average of 8 hours).
Finally, experiment with settings but don't get too hung up on setting that you forget to have fun.