Synthetic Full Backup
When choosing Amazon S3 as your target storage and enabling block-level backup, you can enable a synthetic full backup as well.
This feature is only supported for image-based backups.
A synthetic full backup is a combination of a full and block-level backups. As opposed to a full backup, a synthetic full backup does not upload a full version to the target storage with each backup. Instead, it assembles a new revision directly in the target storage by combining already existing blocks from previous revisions with newly uploaded blocks. This enables you to upload less data to the target storage and speed up the upload process.
With this option enabled, the first run of your backup plan will automatically force a full backup (synthetic backups will be executed during subsequent runs of this backup plan).