Anatomy
An optional leading icon can be added to make it stand-out and/or increase the 'scannability' on the page. Other elements are mandatory.
States
Types
Based on context, there are two different ways of using an expansion panel.
An expansion panel could be displayed as a 'flat' list to create vertically stacked items that fit the content hierarchy of the page. In cases where you need extra emphasis or when you use it as just a single item, you could also apply the 'elavated' type.