All Metal detects anything metal. Zero Discrimination uses the parameters established by the notch segments to categorize each target within one of those notch segments. Recognize that notch segments were established with specific ferrous and non-ferrous properties in mind. On the 705, they called the "most ferrous" notch -8. And they call the most conductive notch +48. In reality, there could be metallic properties that are more ferrous than -8, or more conductive than +48. If you were using Zero Discrimination, you wouldn't detect them. But in All Metal, you would because All Metal is not restricted by the programming used to set the parameters of the notches.
How can we make that work in our best interest......Deeply buried iron is most generally the -8 notch segment. In All Metal, those same deeply buried pieces of iron will be recognized as metal, and generally bounce between -8 and +48, giving that occassional burst of high tone, blended in with the low tones. It may still bounce a bit between the lowest ferrous and the highest non-ferrous notch when using zero discrimination. But generally, zero discrimination offers a more consistent -8 TID on bits of iron than it does in the All Metal mode, without having to reject notch segment 48 to minimize the wrap around. JMHO HH Randy