A lot of MMORPGs have mechanics that demand a lot of skillbars. You might have 30 different skills, plus some consumables that you want quick access to, and maybe some other items that you want to be able to use there. World of Warcraft is the best known example, of course, but there are plenty of others, such as EverQuest II, Final Fantasy XIV, and so forth. Astellia also goes that route.
Having a custom key for every single spot on a skillbar can get awkward, even on a keyboard. Especially if you also want shortcuts to 20 other things to open menus, select party members, and the like, you end up having to use either combinations of keys or having to reach for more distant keys like the F-keys or the 10-key portion of a keyboard. If you want to play using a controller, then even in a game like FFXIV that actively tries to be controller-friendly, some slots that you want to use a lot end up taking combinations of three buttons to activate.
Astellia found a solution to this, as mentioned in the title. Like other MMORPGs, you can have several skillbars on the screen at once, and put lots of skills or items or whatever in them. Unlike any other game I've seen, each slot on the skillbar can have up to four skills in it. If you click on the slot, it opens up to show four spaces to put skills or items or whatever you want. You can put whatever you want in the four slots, with no obligation to use all four, then click again on the slot to collapse it down to a single square.
When you go to press the hotkey, it will activate whichever is the lowest of the four skills that isn't on cooldown. The icon displayed on the hotbar is whatever it will activate if you press the button. For example, when nothing is on cooldown, it will show the lowest skill. If you activate that skill and it then goes on cooldown, then the slot will show the second skill. Pressing the same key again will use the second skill, which then goes on cooldown, and then the slot will show the third skill. If all skills in a slot are on cooldown, it will show whichever will be the next to come off of cooldown, as well as the time until it becomes available again.
In principle, this could create a problem where you intend to use one skill, then another below it in the stack comes off cooldown just before you press the button, causing you to use a different skill. In practice, this doesn't seem to happen much. In some cases, it's because I tend to stack similar skills (e.g., single-target melee damage) on the same slot so that using the "wrong" one isn't such a big deal. You could make it into a problem with dumb choices of which skills to stack in the same slot, but that's fundamentally a problem of user error.
There is no requirement to use all four slots. It's completely fine to have some slots with only a single skill or item, some others with two, and some with all four. The same skill can appear in the stack for multiple slots if so desired, though obviously, using the skill from one slot puts it on cooldown for all of them. A skill that has no cooldown cannot have anything placed above it in the same slot, as anything above it would never be reachable.
One of the great things about this interface innovation is that you don't have to use it if you don't want to. If you'd prefer to reserve a dedicated skillbar slot for everything that you want to use, you can. Nothing is lost by making this into an option.
But if you notice that whenever you use skill A, it's immediately followed by B and then C, you can put A, B, and C all into a single slot. Rather than having to jump from one key to the next to go through your rotation, it's simpler to just press the same key three times in a row, and not have to assign other things to more distant keys that are hard to reach.
The reason I really like it is, of course, that I use a controller. This is the key feature that makes Astellia into the most controller-friendly of the MMORPGs that have dozens of skills for you to use. It's still not as controller-friendly as some games that only have a handful of skills, but that's a substantially different design choice.
And that's why I'd like to see it implemented in a lot of other games. If having 20+ skillbar slots visible on the screen isn't ridiculously excessive for how many skills you have available, then adding the ability to stack skills into a single slot would make the game a lot more controller-friendly.
That said, it's not something that is helpful to all MMORPGs. Some games like Guild Wars 1 or Neverwinter just don't let you have very many skills available. They have few enough that you care exactly which skills are available at a given time and always need direct access to every skill. Meanwhile, they have few enough skills that you don't need such awkward combinations of three or four buttons to do things, but never really need more than two. In some games, you can even get away with having a dedicated button for everything that you want quick access to. But that's a restriction on game design, and stacking skills in a single slot is a way to get around that restriction.