a. In a hot standby system:
The master instance is the database instance in the cluster that is in the ONLINE operational state and with which the users are working. If the master instance fails, one of the standby instances in the cluster becomes the new master instance.
Concepts of the Database System, Hot Standby
Database Manager GUI, Setting Up a Master Instance (Hot Standby)
Database Manager CLI, hss_enable
b. For synchronization of multiple database instances using the MaxDB Synchronization Manager:
The master database instance is the database instance with which the client database instances synchronize their data. The data of the client database instances is generally a subset of the data of the master database instance.
See also: Synchronization Manager documentation