What is an Oracle snapshot standby database?

A snapshot standby database is a fully updatable standby database.

Redo data is still received from the primary database, but it is not applied immediately. Before the redo database is applied, the database will need to be converted back into a physical standby database.

Any local updates that the snapshot standby database may have received, will be deleted when the snapshot database is converted into a physical standby database and resynchronized with the primary database.

Oracle 12.2 Data Guard Concepts and administration documentation

Oracle support Using Snapshot Standby Database. (Doc ID 443720.1)