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Class 1/2/3 items are defined by the modifiers :c1, :c2, and or :c2c3. They read their objects with a class 1/2/3 poll, which returns values for all objects in class 1/2/3 whose values have changed since the last class 1/2/3 poll. The object must be in the class specified by the modifier, and collection of that class must be enabled at the device. Otherwise, responses to the class 1/2/3 polls will never contain a value for that item.
ACM has no way of determining which objects are in class 1/2/3 for any given device. It can’t prevent an item from being defined with the wrong class. It will allow items for the same object to be defined with multiple class 1/2/3 identifiers, even though the item can only be in one of the classes in the device. The user is responsible for determining and defining the correct class 1/2/3 for the object.
Initializing Class 1/2/3 Items
Class Any object in class 1/2/3 will also be in class 0. When class 1/2/3 items will are first activated, they'll be initialized with a class 0 poll (all objects in data integrity check poll, unless the option "No class zero poll to initialize items" is checked in the Device tab.
There's also an option called "Initialize class 1-3 after comm failure". Since class 1/2/3 polls only return values for objects that have changed since the previous class 1/2/3 poll, if the class 1/2/3 items are invalidated after a communication failure, by default they will not become valid again until their values change in the device. Selecting this option will send a class 0 data integrity check poll to initialize the class 1/2/3 are also in class 0), unless the option to disable class 0 initialization is selecteditems after a communication failure, so that all class 1/2/3 items will become valid again immediately once communication is restored.
The Data Integrity Check Item
There's an item called DataIntegrityCheck. This item sends a class 0 poll, and updates all items that are activated for the device with the data from that class 0 poll, regardless of what modifier those items have.
The class 0 polls that are sent on interval and by demand polls for items with :c0 will only update the items with :c0, not items with other modifiers.
The class 0 polls that initialize class 1-3 items upon activation and after communication failure are data integrity checks that will update all activated items.