More Horizontal Progression please

[Ibi]

They would only be horizontal if said boss took the same amount of damage regardless of type.

So a well-balanced proc would be instead of a fireball, it would be a boulder that did physical damage.

Gear A: Adds an extra attack that deals X damage.
Gear B: Sometimes attacks three times dealing damage.
Gear C: Occasionally launches a saw that deals damage.

It shouldn't matter what type, unless all the types are the same. If they are different then its unbalanced procs.

Okay, fine, we'll use your jumping off point instead. The way I see it, there are three possible outcomes here.

Scenario 1: A, B, and C all do effectively equivalent damage on all fights. Maybe slight advantages here and there.

This is not choice. It's an illusion of choice, because while you have the option to use any of them, it also doesn't matter which one you use. Once you've acquired one, there's no reason to go after either of the others unless you're crazy about min-maxing.

Scenario 2: One of the three is the superior choice in all situations.

I think everyone agrees that this would be an example of vertical progression. You have a single best option and, once you have that, you no longer need concern yourself with other options.

Scenario 3: Different options are better for different situations.

Here's where the split is occurring. Most people in this thread would say this is horizontal progression.

You seem to have a further distinction:

Scenario 3a: Specific options are slightly superior in different situations. All options are viable in all situations.

You would call this horizontal progression, as would everyone else.

Scenario 3b: Specific options are marginally superior in different situations. Other options are still viable, but inferior.

You would call this vertical, most everyone else would call this horizontal.

What I'm trying to get you to clarify, and what you're consistently either avoiding or failing to answer, is where you think the line is between 3a and 3b.

What's the difference between a small advantage and a noticeable advantage?