The Basic Edition of DivisiBill is free and does not show ads. It is possible to purchase a couple of additional features on the settings page, both are optional; the app works without them. You can use this page to subscribe to the Professional Edition or purchase packages of OCR scans (for more details on OCR, see below).
The Professional Edition annual subscription enables automated cloud storage and backup for bills, venue lists, people lists, and selected program properties. It will renew automatically once a year until you cancel it. As an alternative you can use the archive and restore buttons on the settings page to back these things up, or rely on Android AutoBackup. The subscription renews automatically unless you elect not to renew it; you can cancel at any time but there are no refunds for unused time.
The settings page for the Professional Edition has some additional options in order to support automated cloud backup. In practice most users can rely on Android AutoBackup to store copies of all DivisiBill data stored on an Android device in their Google Drive account, but the Professional Edition subscription provides additional security (files may optionally be encrypted) and convenience.
OCR stands for Optical Character Recognition. In DivisiBill it is used by a cloud-based service to scan a picture of a bill into item names, amounts, and a few other useful values (tax, for example). It cannot help you distribute the cost of items - only you know who the participants were and what each one had - but it can take a lot of the tedium out of the item data entry process and make it less error prone.
Each OCR scan costs a few cents. You can purchase packages of OCR scans in both Pro and Basic Edition. You will retain any OCR credits through an upgrade or if you uninstall and reinstall DivisiBill.
Modern mobile phones can take very high resolution pictures, which are unnecessarily large and detailed for OCR. DivisiBill reduces the resolution and converts the images to black and white to save storage space and analysis time. These files will be saved along with all the other device state by Android AutoBackup.