Many CitectSCADA items require a cluster to be specified in order for them to work correctly. This ClusterName can either be explicitly specified when the item is called or displayed, or an item can use the default cluster context of the calling process or page. If your system has only one cluster, then no cluster name needs to be specified. See Cluster context rules.
Cluster context, therefore, is the default cluster used for tag resolution and execution of expressions and Cicode functions. In the case of tags, a cluster is explicitly supplied by prefixing the tag name with the cluster name. For built-in Cicode functions ClusterName is usually an optional parameter.
Server processes (Alarm, Trend, Report) have their default cluster context set to their own cluster, so that, for example, Alarm definitions that contain variable tags without clusters explicitly supplied will attempt to resolve those tags to their own cluster. Cicode tasks started from server code will inherit the servers cluster unless one is explicitly given.
Graphics pages have no cluster context by default. A page's cluster can be supplied statically in the page appearance properties, can be inherited from a previous context, or can be applied dynamically using an optional parameter in the Cicode function that displayed the page. Any Cicode tasks started from a page will inherit the cluster context of that page.