Quote Originally Posted by Sandpark View Post
Horizontal Example: You get a ring with initial stats usable at lvl 15. The stats increase at 20 and again at 30.
Vertical example: You get a ring with initial stats usable at level 15. Then need to own another at level 20 and another at level 30.
That's not horizontal progression, that's just gear scaling. You can have scaling without horizontal progression and visa versa.

Horizontal:

There are two level 50 tank chest pieces. They're all roughly the same in terms of power, but each is situationally superior to the other.

Let's say Chest A has a ton of accuracy. Chest B has a proc that gives you a shield for 1 second every time you're hit.

Now let's make up two bosses. Boss 1 is very evasive and attacks slowly. Boss 2 is not particularly evasive, but every time it attacks it swings twice. Chest A would be much better against Boss 1, but the accuracy would be wasted on Boss 2. Chest B would let you shield half of Boss 2's attacks, but the proc would be useless on Boss 1, who attacks too slowly.

In the next round of content, there would be more bosses added for which A or B might be superior, or perhaps new chests C or D, along with new items that could be superior for Bosses 1 or 2, or perhaps new bosses 3 or 4. A and B likely remain competitive options though, even once you have C or D.

Vertical:

There are two level 50 tank chest pieces. They're all roughly the same in terms of power.

In the next round of content, chests C and D are added. They're of a higher item level than A and B. Neither A nor B is likely to be relevant once you have C or D (barring constraints like accuracy).