When a Standard method backup runs into a file that is open or 'in use', it will *skip* backing that file up and move on to the next file selected for backup.
When a VSS method backup runs into a file that is open or 'in use', Windows will use VSS to put a *copy* of that open file into a VSS shadow copy. Then once Windows has successfully created a shadow copy with all of the files that you'd like to back up, that shadow copy is then passed to NetVault for backup.
So simply put, the VSS method will allow the back up of files that either end users or the OS itself has open, while the Standard method will skip those files.
And any files that the Backup skips will not be available for Restore, if they are later needed.
