Think of it this way: Prospecting Mode is virtually the same as All Metal Mode but with an "Iron Masking" feature.
Prospecting does seek slightly deeper than Coin & Relic mode. You can discriminate iron targets in Prospecting mode and the default setting is usually fine to start with although you may wish to increase the masking strength slightly. Try and experiment as you go. In Prospecting Mode, the detector will identify virtually any metal targets. When attempting to ID the targets you find (you'll need to switch to Coin & Relic Mode to do this), the low tones (eg -2 -4 -6 etc) tend to be iron or junk such as nails and horseshoes etc. Higher tones (+4 +12 +6 +22 etc) tend to be legitimate targets such as coins, non-iron artifacts, bottle caps (okay, that's not a good target), foil, gold or silver. Using a 3-Tone effect is a good idea... that way you'll know by sound if you've struck a positive target.
When you find a target using Prospecting Mode, you can then switch to Coin & Relic or All Metal mode to try to ID the target before you dig it.
Cheers,
Marco