You can also click the third tab which takes you to the Custom Fields configuration for this Subscription. Note that the Custom Fields that are available on this screen must have been configured in the underlying Plan first; the Subscription simply inherits these Custom Fields and allows you to set values on them that are specific to this customer and their Subscription.
To configure custom fields on a Plan or Product, see the section Adding Custom Fields to Existing Plans and Products.
You can manage and configure the custom fields for the Subscription as a whole (i.e., at the Plan level), as well as any custom fields that may have been configured for individual Products within the Plan. Choose which fields to manage in the dropdown at the top of the screen.