Stimulus items are used to support your questions. You might create a stimulus component that has text describing a scenario. They might also be instructions to take the test or pieces you'll use to customize the look of the test, like a welcome message, a banner, or a logo.
Question items are used to ask for a response from test takers. 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.