I think you’re confused as to what exactly these things are doing. Yes, you can set the stacking bonus to 0, but that won’t do anything.

More than likely. I've been trying to find a way to enable stats going above 20, maybe even limiting them on a per-player basis for some situations, and so on. but every time I look for it, I can't seem to find the right information. 

I tried with just the maximums, and my stats were maxed at 20, no matter what I did. 
I add the stacking, and it works as intended, as long as I remove the item that gives +10. so I am just confused where I got it wrong.. 

It takes a combination of modifiers, the Ability Score Maximum Modifier and the Stacking Bonus Modifier. You need both. The Ability Score Maximum Modifier increases the max ability score above 20, and the Stacking Bonus Modifier increases the score to a number capable of exceeding 20.

So if the character has a score of 18 and you increase the maximum by four and give them a stacking bonus of four, then that will raise their maximum score from 20 to 24, and raise their current score from 18 to 22. If they then later, increase their own score by two points to a maximum of 20, the stocking bonus will kick in and raise it to the full 24. Does that make sense finally?

Yes, that's not the part that was confusing me, I was already understanding that. What I'm wanting, is the ability to use the 'stacking' effectively, in a way that allows a player to set their own bonuses. 
Such as from the use of the Ability Score Improvement choices from class levels, or feat choices. [Ex: Selecting between Str and Dex] 

Effectively: If the player is already at 20, and they want to improve their dex by another 2 points, I want them to be able to do so, up to whatever hardset limit I put in, without using the 'customize' part of their character sheet, or me having to create them a brand new feat/item every time. 

im about to be the annoying person to reawaken your thread but THIS is so helpful, but I can't seem to find the set -> subtype: ability score maximum
did dnd beyond decide that this is not allowed anymore or did i possibly miss something?

nope. i found it. two seconds later <3 sorry for the notif i love you guys for figuring this out

am also trying to do this

I'm just going to necro this baby up and mention that when you stack magic items with "Ability Score Maximum" increase, those increases STACK as well, which they shouldn't (or at least, there should be an option to specify a set limit, or a continual stacking increase).

By this, I mean if you have two items that had "+2 to Strength, this item can't increase your maximum above 22", and you already had a 20 strength before equipping multiple of these items, every time you equipped another, your max would increase by two every time, easily breaking the mechanics of a 22 max (within DnDBeyond). Kinda suck, I wish you could just put in "22" as the score max, and it would respect every instance of an item that does that. 

Quote from IamSposta >>

You have to select an Ability Score to modify each time you add the Modifier. Like this:

Modifier: Set->Subtype: Ability Score Maximum->Ability: Str->Fixed Value: 4

That👆Modifier will set the character’s Maximum Strength score to 24.

Modifier: Set->Subtype: Ability Score Maximum->Ability: Cha->Fixed Value: 6

That👆Modifier will set the character’s Maximum Charisma score to 26.

 just to point out, I dont have Subtype: Ability Score Maximum under the Set dropdown.  I have Ability Score Maximum under the Bonus dropdown.

Quote from trotdawg2k >>

Quote from IamSposta >>

You have to select an Ability Score to modify each time you add the Modifier. Like this:

Modifier: Set->Subtype: Ability Score Maximum->Ability: Str->Fixed Value: 4

That👆Modifier will set the character’s Maximum Strength score to 24.

Modifier: Set->Subtype: Ability Score Maximum->Ability: Cha->Fixed Value: 6

That👆Modifier will set the character’s Maximum Charisma score to 26.

 just to point out, I dont have Subtype: Ability Score Maximum under the Set dropdown.  I have Ability Score Maximum under the Bonus dropdown.

 I recieved a message from iamsposta saying that I was correct in that the DnDBeyond site changed the Set-->Subtype: Ability Score Maximum has been moved from the Set dropdown, to the Bonus Dropdown menu since the thread first was started.