What is a "QUIESCED mode"

Quiescing
Quiescing is the mode that transfers a replication environment from the normal mode to the quiesced mode. While a replication environment is quiescing, the user is no longer able to execute a transaction against a replicated object, but any existing deferred transactions are propagated. Queries against a quiescing table are allowed. When all deferred transactions have been successfully propagated to their respective destinations, the replication environment proceeds to the quiesced mode.

Example : Suspending Master Activity
Whenever some administration tasks needs to be taken, such as adding an object to a replication group, you must suspend master activity for the nominated group. This will put the group on the master and master definition site into QUIESCED mode. At this stage, only SELECT statements are allowed on the objects within the group. Any other DML or DDL statements will generate ORA-23311 or ORA-23326.

ORA-23311 object group “name” is quiesced
Cause: SUSPEND_MASTER_ACTIVITY has been called before the object group has resumed normal operation.
Action: If a RESUME_MASTER_ACTIVITY request is pending, wait until it completes, and then reinvoke SUSPEND_MASTER_ACTIVITY.

ORA-23326 the system is being quiesced
Cause: A deferred remote procedure call operation was attempted while the database was quiesced.
Action: Resume database activity with the DBMS_REPCAT.RESUME_MASTER_ACTIVITY call.
For example,

How tp put system into “QUIESCED mode”
exec dbms_repcat.suspend_master_activity(‘BILLING_RUN’);
You must issue this command only on the master DEFINITION site. All master sites involved in the group will be QUIESCED accordingly.