Where is that at? I've never heard of the use a feature as an alternative action
It is in the artificer's class feature.
While holding the object, a creature can take an action to produce the spell’s effect from it, using your spellcasting ability modifier.
There are a lot of features that add alternative actions, bonus actions, and reactions. In fact every bonus action that isn't 2 weapon fighting or a spell and every reaction that isn't an attack of opportunity or a spell was probably granted by a feature or trait. There are more universal actions, but still plenty granted by features: certain channel divinities, wild shape, and primeval awareness to name a few.
Any time an object requires an action to use and it's not a magic item, it falls under [Tooltip Not Found]. A spell-storing item 1) is an object, 2) requires an action to produce the spell's effect and 3) is not a magic item, so it should fall under Use An Object.
I don't know if "can take an action" vs "as an action" have any differences as far a rule interactions.
On top of that the class feature creates the item but the item can be used by practically anyone. I don't know if I would consider the SSI the feature anymore than than a wizards spell book is one. Both are created by a class feature but are independent items after the fact.
Think about it this way. Does the item have any properties that no longer rely on the feature? For example infusions turn items into magic items, those magic items have functions not dependant on the artificer's feature. What can a non-magic weapon be used for without having to follow the feature?
Also, the feature does not say the item can be used to produce the spell effect. It specifically says action. The artificer does some weird stuff that the rules don't talk about, like giving away its class features. So all we can really go by is the specific wording of the feature.
It is not an [Tooltip Not Found] action, it is an "use feature" action, or rather an action granted by a feature. No fast hands or haste.
You are right about it not being a cast a spell action though, that means you can cast bonus action spells on the same turn.
Which spell storing items do you consider to not be magical items? I think I'd automatically consider any such thing as magical.
The feature does not make the item magical, so it isn't. It is the artificer's magic.
It is in the artificer's class feature.
There are a lot of features that add alternative actions, bonus actions, and reactions. In fact every bonus action that isn't 2 weapon fighting or a spell and every reaction that isn't an attack of opportunity or a spell was probably granted by a feature or trait. There are more universal actions, but still plenty granted by features: certain channel divinities, wild shape, and primeval awareness to name a few.
Any time an object requires an action to use and it's not a magic item, it falls under [Tooltip Not Found]. A spell-storing item 1) is an object, 2) requires an action to produce the spell's effect and 3) is not a magic item, so it should fall under Use An Object.
The Forum Infestation (TM)
Think about it this way. Does the item have any properties that no longer rely on the feature? For example infusions turn items into magic items, those magic items have functions not dependant on the artificer's feature. What can a non-magic weapon be used for without having to follow the feature?
Also, the feature does not say the item can be used to produce the spell effect. It specifically says action. The artificer does some weird stuff that the rules don't talk about, like giving away its class features. So all we can really go by is the specific wording of the feature.