Field attributes are the attributes that can be applied to fields of an array, structure, or union: ignore, size_is, max_is, length_is, first_is, last_is, switch_is, and string, and pointer attributes. For example, field attributes are used in conjunction with array declarations to specify either the size of the array or the portion of the array that contains valid data. This is done by associating another parameter, structure field, or a constant expression with the array.
The ignore attribute designates pointer fields to be ignored during the marshalling process. Such an ignored field is set to NULL on the receiver side.