When urbackup makes a image backup (not sure about the file backups, since i don’t use those), it will scan the blocks in the server for any changes based on the last backup made. It always creates a incremental backup with only the changes since the last backup made stored in the file.
If you so specify, it can then turn this incremental backup into a full backup, merging the last full backup and incremental backups after that into one new full backup.
The main difference in full and incremental backups is size and speed. A full backup is fastest to open/restore as the software only needs to open one file. If you specify one full backup every 30 days and make a incremental every day the software needs to open the last full backup as well as all incremental backups to check for changes since when restoring. This takes more time. Incremental backups can be very small though and great for increasing backup retention while not consuming so much disk space.
The scan time is not affected by the full/incremental backups as all full backups are in essence incremental backups that have been converted to full backup after the backup is done. scan time is only to check the blocks in the volume for changes. the CBT client can keep track of which blocks have been changed to speed up the scan time as it will only need to scan the blocks that have potentially been changed.
I don’t fully understand the deduplication process as my full backups are actually the size of full volume backups, but it might be specific to the storage formatting you use (ZFS for example). Someone else will need to help explain this.