The practical upshot of this is mostly "you can't both prevent something from happening and do extra stuff because it's happening" – e.g., if Volcano Looming High's Presence would be destroyed, but it's saved somehow, you don't get to deal damage.
If a modifier cancels just
part of something, modifiers to that part don't apply. Eg:
"Entrap the Forces of Corruption" doesn't stop Blight from being added, but it does stop it from cascading, so another modifier which changed how cascades worked wouldn't apply.
As before, if you have multiple modifiers with no clear order, you get to pick which happens first. For instance: you play
"Infestation of Venomous Spiders" with threshold on a land with Dahan about to Ravage. A Fear Card says "Invaders do not Ravage in lands with Dahan". When that Ravage would happen, there are two different "skip this Ravage" modifiers. You get to decide which one to do first, so can choose it to be the Venomous Spiders. This wouldn't be true if the entire Ravage Step got skipped, or if the Ravage Card were changed to match different lands – those happen before you ever reach the individual Ravage Action – but as of
Jagged Earth, the only Fear cards that work this way are
"Explorers Are Reluctant" and "Immigration Slows", levels 2 and 3.