Like conventional records in Fulcrum, child records - which are created from repeatable sections - can use up to 5 fields that are contained in the repeatable section to create a child record title.
To create the child record title, select the repeatable section on the App Designer page. The field settings for the repeatable section will be displayed in a modal window. There, click the 'title' button. A new modal will now be displayed allowing you to select what fields you would like to use to create the record title for the child records produced by this repeatable section.
Repeatable sections are only available in some plans. Please check out the pricing page to see which plans have access to repeatable fields.
Notes
If you change the title after data has been collected, you will need to reset the record cache and then run the sync on the mobile devices. See this article for more information.