All items are General when created. If you add a legal notice or a hyperlink to your item, it remains a general item because those are both general components. But depending on the components you add to the canvas, the type might change to Stimulus or Question.
General and Stimulus items are used to support your questions. Adding a stimulus component changes the type to Stimulus. You might create a stimulus component that has text describing a scenario. General items might be a legal notice or pieces you'll use to customize the look of the test, like a welcome message, a banner, or a logo. Functionally, General and Stimulus items are the same.
Question items are used to ask for a response from test takers. Adding a response component changes the type to Question. They can be linked to a CompetencyCore item if you want to add them to a test that provides detailed scoring. Select a competency or knowledge area and the level you're evaluating. You could create a question with a multiple-choice component testing the user's understanding of a scenario described in one your Stimulus items.