I could be wrong as I used to have a Tejon and also my Shadow X5 and they seem to work the same and both a excellent detector. The Disc is the main disc is the normal disc which is set low to hear anything other than iron, the alt disc is actually like a higher disc to tell if it is a copper or belter coin , on my X5 it is called coin check. This way you are detecting for anything other than nails and if the signal sounds good you switch over to the alt disc and it still reads good you have a higher conductivity item. Like I say I could be wrong, but think this is how it works on the Shadow X5 and the Tejon, its like a second disc, at the lower setting it says it is a metal item better than a iron nail, so you switch to the alt disc and it will tell you if it is higher than what that disc is set at. When I had mine I set the reg disc to lower than a nickel and the alt disc set to just start breaking up a pull tab. I would swing away and when I got a solid signal I would switch to the alt disc and it it sounded good I would dig. If it didnt and sounded good in reg disc it could be a nickel or even a pulltab and depending on how it sounded. Some of the other Tesoro's Umax some would just turn the disc knob and do the same thing, but feel it took longer.
Good Luck and hope that helps you out.
Rick