snapshots are taken before backup, and deleted after backup.
Where cache for changed blocks are placed is depend on snapshot method(VSP/VSS) and settings.
See Administrator's Guide page 387 for detail.
support.veritas.com/docs/290203
When you are talking about snapshot of snapshot client, see Snapshot Client Administrator's Guide and find which type of snapshot will realize you needs.
support.veritas.com/docs/290224