I borrowed this illustration and the explanation below from another forum post by longhair. He and Randy "digger" Horton are the X-Terra gurus. It was much easier to copy/paste this than me writing an essay to explain it.lol
"It has to do with target bin segment widths assigned. Low frequency coils will be more accurate on high conductors (copper, silver, brass), and high frequency coils will be more accurate on low conductors (gold, pewter, iron)."-longhair
F5 is a good bang for the buck MD. Big display! I really like having all the info on one screen as I hunt. Just too chatty for me when EMI becomes an issue. I like the auto noise cancel feature on the X-terra 705. I have hunted directly under power lines with no issues at all. The frequency shift setting on the F5 helps some but you will still be turning the sensitivity down to deal with any EMI, even from other nearby MDs on a group hunt.