In general, Microsoft VSS framework coordinates Snapshot copy-based backup and restore operations and includes these components:
MS Framework:
VSS Requestor = The VSS requestor is a backup application, i.e SnapCenter-SQL_Plugin (Application specific).
VSS backup application will explicitly select the databases from the metadata that is returned from the "SQL writer". The snapshot set should include all the volumes necessary to back up those databases.
VSS Writer = The SQL writer is a VSS writer provided by the SQL Server. The VSS writer that owns and manages the data to be captured in the Snapshot copy, i.e SQL_Writer for MSSQL.
VSS provider = The VSS provider is responsible for creating and managing the Snapshot copy, in this case Data_ONTAP_VSS_Hardware_Provider. (To ensure that the Data ONTAP VSS Hardware Provider works properly, do not use the VSS software provider on Data ONTAP LUNs.)
In order for Application-consistency: VSS Requestor coordinates with VSS, which in turn coordinates with Writer (SQL Writer) to ensure write requests are frozen and write caches are flushed before you make backup copies. If there is no plug-in, this will not be coordinated and you may end up taking VSS file-system snapshot, which could mean the backup is not 100% application consistent and may not be able to restore to the same state (Especially for the DB files hosted on the NetApp LUN.)