This is the default mode, and should be used when your product configuration has items or descriptions that are important to visitors.
You should always use this mode if you are offering items as part of a product offering and if visitor can optionally alter the QTY of each item. The expanded mode ensures the visitor can see all the options before proceeding.
Expanded Online Store example:
Use this mode when you are offering basic product configuration, for example if you are not including any items or are enforcing that all attendees get the item then this would be a good choice of display mode.
Keep in mind a user can press "Show Full Details" to expand the detail, so when using this Display Mode, you're still required to provide description fields.
Compact Online Store Example: