On the upcoming Dark Harvest nerf, and ? on the mechanics.

So this is what the notes for patch 8.24 look like for Dark Harvest

Balance Changes in 8.24

Runes

Dark Harvest Nerf - Dark Harvest's still looking too universally effective and on too many champs too powerful. We'll be cutting its base damage (20-60 instead of 40-80) and making it so it can't trigger off proc damage to make it less in-discriminant. Hitting base in particular to reduce its lane bullying potential, would like picking a scaling rune like this to come with more early game trade off. Once those changes have hit we'll then see if we should also be considering changes post 8.24 to the reset (should it be longer, or potentially removed?) and health threshold for the trigger (would 40% instead of 50% be more appropriate?). Don't want to hit too much at once though and overcorrect, hence a couple of changes next week with further follow up if needed.

So my question is, what does, "making it so it can't trigger off proc damage" mean? Is this referring to rune its self? Or are there other kinds of proc's in the game that this is referring to? I am still trying to learn about all the different kinds of damage in the game since I still consider myself a noob more or less. I would also think that would make it a lot harder to tell when you're going to get a stack off Dark Harvest and when you're not. While it looks like something does need to be done if this is the way it's going to work I'm not much a fan of it going this way. It more or less makes it less skill based when you see that life chunk off and he falls to 40% HP but then you find out you actually didn't get the proc.

I mean, basically, if you hit a Karthus ult what % of that Ult is from Dark Harvest and when was that damage applied? And since the more stacks of Dark Harvest you have the higher the damage from Dark Harvest, the harder it would be to be able to tell when you are going to get a stack, due to the change in damage modifier.