Does this feature on a m/d cancel steel or iron up to a certain size or does this feature differ between different m/d's or does it just block the signal for all steel or iron?
What detector and what feature?
Nothing really cancels out masking by iron~ or other metals for that matter. Some detectors can somewhat separate iron signals from other metals signals.
I use moderate IM -10 when coin hunting/jewelry hunting and I still dig iron nails and pieces of iron. But sometimes when I get a good signal - there is iron in the hole along with an old coin.
So the iron does not mask the fact that silver is below the coil too when using iron mask on my machine. Least that's how I look at it. Maybe that's not how Minelab looks at it. No idea.