In a hot standby system, one or more standby instances regularly track the data changes that have been made in the master instance. If the master instance fails, one of the standby instances becomes the new master instance. Standby instances are always in the STANDBY operational state.
Concepts of the Database System, Hot Standby
Database Manager GUI, Adding a Standby Instance (Hot Standby)
Database Manager CLI, hss_addstandby