The Back and Done buttons are found on the top menu bar of the Data Window

Normal Operation

The Back button is primarily used to leave a section and save changes when you don't yet wish to complete that section.

The Done button is used when you're happy that you've completed a section and wish to check for completeness, or think of it as 'signing off' that section. 

Back

Pressing the Back button will take you back up the previous level of the form, or if you’re already at the top level it will take you back to the Reports Window. If you’ve made any changes, you’ll be asked if you want to save these.

Done

Pressing the Done button also takes you back up to the previous level of the form. However, by pressing the Done button, you are telling the App that you’ve completed that section of the form. The App will therefore automatically save any changes you’ve made, and will also check that all mandatory fields have been completed. If there are any mandatory fields which have not been completed, then you’ll be shown a message about these.