Groups and Items within Groups have an Active Flag. The active state of the group is maintained separately from the active state of the items. Changing the state of the group does not change the state of the items.
For the most part the Active flag is treated as `abstract' within this specification. The state of these flags affects the described behavior of various interfaces in a well defined way. The implementation details of these capabilities is not dictated by this specification.
In practice it is expected that most servers will make use of this flag to optimize their use of communications and CPU resources. Items and Groups which are not active do not need to be maintained in the CACHE.
It is also expected that clients will simply set and clear active flags of groups and items as a more efficient alternative to adding and removing entire groups and items. For example if an operator display is minimized, its items might be set to inactive.
Refer to the Data Acquisition and Active State Behavior summary earlier in this document for a quick overview of the behavior of a client and server with respect to the active state of a group and items.
OnDataChange within the client's address space can be called whenever any active data item in a active group changes, where "change" is defined as a change in value (from the last value sent to this client), or a change in the Quality of the value. The server can return values and quality flags for those items within the group that changed.