Metal detectors, detect conductive materials, metal. That's all you can be 100% sure of, and even then, I think I've been fooled a few times by oak tree roots. When you discriminate, or rely on TID numbers, you are potentially passing up potentially good targets, to save time and effort. I hit my yard a lot, it gets hot around here, and I know I probably don't want to stay out very long, or take breaks in the cool AC. Dark clouds, can mean rain at any time, a light sprinkle or a torrential downpour, never can be sure, so I play safe. I started out, going after the good stuff, discriminated out iron, foil, zinc, and passed up signals that didn't lock into something good. After exhausting the sure things, I backed off the discrimination, and started digging the confused signals. Be running no discrimination and all metal, past couple of months, digging every thing that repeats, and not solid iron, or huge. Unbelievable, how many pounds of nails I've pulled out of the ground. Been using a strong magnet, more than the Propointer, but still finding coins, mostly pennies, even a couple of wheats. My property has been search many times, thoroughly, but still produces.
Discrimination and TID are shortcuts, basically statistical guesses. The detector can only measure electrical characteristics, not atomic characteristics. There are 16 different flavors of iron rust, and they will show up differently. You can mask them all out, but you are also masking out anything similar, electrically. Really up to the individual, pounds of nails, and only a few pennies, some brass and copper junk. I keep looking, because I've yet to dig silver in my yard, but have found 21 wheat, a few other coins in the silver range or close, several other older relics, that leads me to believe there should be some silver around, just probably too deep to detect or dig. Pretty sure there is a bout 5-6 inches of fill dirt, because underneath is the where the many nails are found.