Hello. Thank you for contacting the Toolset support.
Well - within repeating field group you can create multiple custom fields and the custom field key will be provided by user.
In the database - those custom fields are saved with postmeta table with "wpcf-" prefix. So for instance, if you have created custom field with slug "bookname" within your repeating field group, within the database the values for this custom field will be saved with postmeta table with the key "wpcf-bookname".