Information about when to choose the quiesce option for backups
UVB and PHDVB Backup can request quiesced snapshots of your VMs - an option available when you create a backup job. Quiesced snapshots attempt to send special instructions to supported applications to cause them to save their state in a crash-consistent manner. Many active, transactional applications, such as SQL Server, may require quiesce in order to guarantee consistent restores. Most other applications will not benefit from quiesce.
When a backup is taken with quiesce enabled, the operating system and application data are prepared prior to the execution of the snapshot. This is done in an attempt to avoid data corruption. By stopping (with quiesce) any running processes on the guest operating system, the file system contents are kept in a known, consistent state when the snapshot is taken.