VSS完整备份（FULL BACKUP）的定义：

VSS_BT_FULL     Full backup: all files, regardless of whether they have been marked as backed up or not, are saved. This is
the default backup type and schema, and all writers support it.

Each file's backup history will be updated to reflect that it was backed up.

VSS副本备份（COPY BACKUP）的定义：

VSS_BT_COPY     Files on disk will be copied to a backup medium regardless of the state of each file's backup history, and
the backup history will not be updated.


A requester can implement this sort of backup on a particular writer only if it supports the
VSS_BS_COPY schema.

If a requester's backup type is VSS_BT_COPY and a particular writer's backup
schema does not support that sort of backup, the requester will always perform a full
(VSS_BT_FULL) backup on that writer's data.

区别主要在VSS完整备份会更新file backup history并标记为已备份（Each file's backup history will be updated to reflect that it was backed up.），VSS副本备份不会更新file backup history（the backup history will not be updated.）。也就是说

VSS完整备份=VSS副本备份+file backup history标记为已备份

在实际应用中，当采用VSS完整备份时，备份完成后，所有的file backup history都被标记为已备份，其他的备份软件因为识别到file backup history不一样了，会采用不备份或者重新开始一个完整备份的方法来处理，所以当你同时采用了其他的备份软件时，最好用VSS副本备份。

还有一种情况是有一些软件会因为收到一个完整备份成功的信息，而删掉自己的一些log，譬如Exchange：

Only full backups should be taken. Log truncation will occur only after a successful completion of a VSS full backup of a volume or folders containing an Exchange database.

当然这是软件自己对log的策略问题，Exchange可能觉得已经有一个FULL BACKUP了，log也没什么用了。

参考链接：

VSS_BACKUP_TYPE enumeration (vss.h)
https://docs.microsoft.com/en-us/windows/win32/api/vss/ne-vss-vss_backup_type

Using Windows Server Backup to back up and restore Exchange data
https://docs.microsoft.com/en-us/exchange/high-availability/disaster-recovery/windows-server-backup?view=exchserver-2019