Explanation
A synthetic full backup is a backup method that describes copying only changed files each time it runs and using only a single file during a restore. A synthetic full backup is a backup approach that involves creating a new full backup by using the previous full backup and related incremental backups. This means that a backup solution does not have to transfer the full amount of data from the source machine and can synthetize the latest incremental backups with the last full backup to create a new full backup. This reduces the backup window and network bandwidth consumption. During a restore, only the latest synthetic full backup file is needed to recover the data. Open file backup is a backup method that allows backing up files that are in use or locked by applications. Full incremental backup is a backup method that involves performing a full backup first and then backing up only the changed files since the last backup. Full differential backup is a backup method that involves performing a full backup first and then backing up only the changed files since the last full backup.
References: https://www.nakivo.com/blog/what-is-synthetic-backup/
https://www.howtogeek.com/192115/what-you-need-to-know-about-creating-system-image-backups/