|1. When a Differential Backup is performed, it will save the file each time until the next full backup is due.
Note: In order to recover from the differential backup, it requires restoring the differential backup and the latest full backup. This usually takes less time than a full backup. The restoration may require fewer tapes than an incremental backup because only the full backup tape and the last differential tape would be needed. One disadvantage of differential backups is that restoration may take longer to complete than incremental backups because the amount of data changes each time keeps increasing since the last full backup.
See Data Recovery Strategy for the rest of the categories.
If a file is changed after the previous full backup, every time when a differential backup is performed it will save the file each time until the next full backup is due.