The 18 Greens application does not collect any type of personal data.
The 18 Greens application syncs data across devices by the use of your private iCloud Key-Value store. This is a mechanism provided by Apple to securely save application data in an iCloud account with one device and retrieve it on another based on your Apple Id. If you have questions on this please refer to Apple's support documentation on the subject.
The 18 Greens application runs self-contained on the device and does not share anything with other applications on the device. Any data entered into the 18 Greens application by the user, is only used within the 18 Greens application itself.
When the 18 Greens application is deleted from a device, all stored data on that device is deleted. The iCloud Key-Value store will persist based on Apple's iCloud retention policies. You will need to cancel your subscription as this is not done automatically on application deletion.
The developer of 18 Greens, Thumbs Up Application LLC, may use Apple provided analytics and logs to make the application better, more stable, and/or more enjoyable to use.
Once again, the 18 Greens application does not collect any type of personal data. Any data entered by the user is only used within the 18 Greens application.