The $PERSISTENT page source has three elements (see pane at right in screenshot below).
•ServiceAvailable: which holds text that indicates whether the In-App Purchases service is available on the device
•ProductCount: which holds the count of available products
•PurchaseCount: which holds the count of purchases
The values of these elements are each displayed in a label, done by setting that element as the page source link of the label. Each label is next to a button in the blue table. When a button is clicked, the corresponding node in the $PERSISTENT page source is updated, and the updated node value is displayed in the label corresponding to that button.