n is either 1 or 0.
When JoinMirror is enabled (n = 1), the mirror configuration is processed and the instance is considered a mirror member according to its configuration.
When this parameter is not enabled, the mirror configuration is ignored and the instance is not initialized as a mirror member. This is recommended when there is a problem in the configuration which prevents the instance from starting, or if the member must be reconfigured before joining the mirror. For example, if an instance was the primary but no longer is, prevents the system from joining the mirror when it restarts and attempting to become the primary again, which could result in dual primaries.
Changing This Parameter
You can change JoinMirror in the Config.MirrorMember class (as described in the class reference) or by editing the CPF in a text editor (as described in the Editing the Active CPF section of the “Introduction to the Configuration Parameter File” chapter in this book).