Enable server update when data changes
Subscription can be 'on' or 'off'. If Subscription is 'on', server update notification is enabled for the group. The update occurs when the server cache quality or value of the data associated with a daitem object contained by the dagroup object changes. In order for the server cache to be updated, the percent change in the item value must also be greater than the value specified for the DeadbandPercent property.
A Subscription value of 'on' instructs the server to issue data change events when items in the group are updated by the server. Additionally, if an callback function file is specified for the DataChangeFcn property, that function executes. If Subscription is 'off', the server might still update item values and/or quality information, but no data change event is generated.
Note that the refresh function is a special case of subscription, where refresh forces a data change event for all active items.