Create assignments with different type of submissions: onlinetext, files, require click to submit. Check that you can create new submissions in offline if the data is prefetched. Check that you can edit in offline submissions created in offline, but not submissions created in online. If an assign requires click to submit, check that it can be done in offline (it doesn't matter if your submission is online or offline). Check that the offline data is synchronized successfully with the site. Now create a new submission in offline and save it. At the same time, in Moodle web create a new submission too. Synchronize the data in the mobile app and check that the offline data is discarded.