Snippet from the "NetVault Backup Plug-in for SharePoint" :
Every backup stores information, called a backup package, in this directory.
The backup package for a Full Backup must be available for a Differential
Backup to be performed because a Full Backup is the base of all subsequent
Differential Backups. For this reason, Quest Software recommends that you
use the same directory for every backup in a backup sequence and that you
clean out this directory before you perform a new Full Backup.
Check the user guide that can be downloaded from the support portal to know more about the temporary directory.
https://support.quest.com/netvault-backup/9.2/technical-documents
Netvault Backup Release Notes and Guides