It's actually more complicated and bad than you think.

1.What you think happens. You spin around and just land on what you want to use and it happens. I.e. I am low on health..I select a potion..my health should go up.

But it doesn't..

Instead..what happens is..

When you're maneuvering through the circle all you're doing is selecting which item you want to hot key for quick access when you press up the dpad out in the field.

Up on the directional pad can only use one item at a time so they decided to give you a easy list of things to put on it. I.e. That circle

So you've assigned all these different potions and things to your wheel.

When you press up.(and keep pressing up)on the dpad it will bring up the radial wheel with all the things you assigned.

When you roll around with the right stick and highlight the one you want and let go, you're not actually..using the item. You're only saying you want that to be shortcutted when you're in the middle of battle and you press up.

Example. You're in battle and low on health .,you press up and use a potion. But then a minute later you're short on magic....but if you were to press up you'd technically use a health potion still

Going to that radial wheel and then selecting a magic potion would replace that health potion with this new one

Selecting in the radial wheel doesn't use the item. It just makes it so you can use that item in battle when pressing up on dpad

Okay. Well that's utterly ridiculous. I mean, "assign a quickslot" kind of does give the impression that when you've assigned something you can bring up the menu and use it.

And now I can't even log in to the game. I shouldn't have ignored those bad reviews.

Basically any item can be quick slotted, simply enter your inventory select the item, and hit the 'assign' option, you can have multiple items in your quick slot area. Now the item that is inside that box to the left of your ability bar is the item you have selected, meaning when you hit the up arrow it will use that item. To change the item quick slotted, simply hold the up arrow and the quick slot wheel will appear and you can cycle through them and select the new item you want to be quick slotted. Now remember any item quick slotted can only be activated when you hit the arrow after you select it.

It is a pain but this is where alchemy in the crafting line can help
You can craft multiple effects potions that will cover your needs with one drink
That’s why all the little flowers cost so much in guild traders