The way Veeam's Synthetic Backup works is that after the first full backup, all subsequent backups are incremental, meaning just the changes from the last backup run, forever. Veeam Backup "injects" the changes into the full recovery file (.VBK) and then also saves any data replaced during this process into the reversed incremental changes file (.VRB). The .VBK file is always a full recovery file and the largest file in the directory. The .VBK file also has the most current modified date as it gets updated after each backup cycle.
The .VRB files do not change, as they contain the .VBK data blocks which were replaced by incremental data for that particular incremental backup run. To restore or roll-back to a particular date/time, all related .VRB files are applied to the .VBK file in the required order to get you back to that point-in-time.
The retention policy specifies how many of the .VRB files you want to keep, this also then corresponds to how "far" you can roll-back. If the retention policy is set to 14, then the 15th time the backup job runs it will delete the oldest .VRB file.
Veeam utilized the same metholody for it's Replication except that the changes get "injected" into the .VMDK file on the target, there is no .VBK file. There are .VRB files and this is what allows for replication roll-back.
If you archive the .VBK and associated .VRB files as a set to tape, you can recover them back to disk if needed and import them into Veeam Backup 2.0 to roll-back or recover to any point in time in that set.