Subscriptions charge one or several plans to a customer on a regular basis.
A subscription item connects a subscription to one or several plans.
When your business model uses a baseline subscription and add-on features that top that up on a recurring basis, you specify for them one subscription with several subscription items. Each subscription item being a different add-on that they can add purchase on top of their subscription.
Prices specify what to charge, when and how often. Use tiers to define each pricing tier your product might have. Specify in each price which tier it belongs to.
Each subscription generates an invoice when it's time to charge a customer. It'll have as many Invoice Items as subscription items and automatic-prorations it might include.