Standard Charge
Product Plan Charges of the type Standard Charge will always result in one Subscription Charge and therefore one Invoice Line and one Invoice Line Description, and will always be available for purchase for the subscribers.
Optional Charge (Add-on)
Product Plan Charges of the type Optional Charge will not be available for purchase, unless they are added to the Subscription.
You add Optional Charges to the Subscription, when they are available from the Product Plan
Optional Charges can be added in bounty, which means you can add multiple purchases of the same product, but each with it's own Invoice Line Description, Custom Price, Discount or billing alignment.