If you keep scrolling down the plugin settings page, you will find a section called Other settings. Here you can enable the following options:
Debug log: this is generally useful for technicians and developers. Enable this option to get additional technical details about your PayPal payments when there are any issues with them.
IPN Email Notifications: enable this option if you want to receive an email notification any time a refund, a chargeback or a cancellation is processed from a customer PayPal account concerning orders of your store.
IPN Email Notifcations Email: enter here the email address where you want to receive the IPN notifications, it might be the admin’s or any other email address to be sure you’re always up to date about what happens in your store.
Invoice Prefix: enter here a prefix that will be attached to all payments coming from this shop to the associated PayPal account. This will help a lot especially if you are using the same PayPal account in more than one store. You will be able to track transactions coming to your account univocally.