Item definition that has this flag set should be ignored by any processing.
The ignored item is still part of the schema. Item instances may appear in
the serialized data formats (e.g. XML) or data store and the parser should
not raise an error if it encounters them. But any high-level processing code
should ignore presence of this item. E.g. it should not be displayed to the user,
should not be present in transformed data structures, etc.
Note that the same item can be ignored at higher layer (e.g. presentation)
but not ignored at lower layer (e.g. model). This works by presenting different
item definitions for these layers (see LayerRefinedAttributeDefinition).
Semantics of this flag for complex type definitions is to be defined yet.