Subscribers are charged based on the quantity associated with a particular plan or addon for a specific subscription for a specific period. Each plan may have one or more add-ons associated with it. You can define the addons when creating or editing plans.
If a subscription has a plan and an addon with the quantity of 10 and 20 respectively, it will show quantity 10 for the plan and 20 for the addon in the metric.
Up: Good
You could alter the quantity for a specific plan or addon and provide volume-based discounts on unit price. This discount will attract new customers without modifying your revenue for a period.
Click here to read about other metrics.