When a data type template identifier is shared among multiple devices but their contents are different, the 'iedType' attribute as described in clause 9.5.1 can be used to identify which data type templates are for a particular IED. If the 'iedType' attribute is not used, potentially, the data type templates can have the same id (with different contents) and therefore will fail the validation against the schema rules.
For example, validating the following xml syntax will fail because the LNodeType is not unique within the file (see the uniqueLNodeType constraint in clause 9.5.1)....
But it fails on the LNodeTypeKey constraint (see A.6). This key constraint states that a data type template must be unique with respect to the 'id' and 'lnClass' attributes whereas the uniqueLNodeType constraint states that the LNodeType element (which is a type of data type template) must be unique based on the 'id' and 'iedType' attributes - but not the 'lnClass'. Therefore, it seems that it is not possible to utilise the 'iedType' attribute as specified in the standard due to this conflict in the rules.
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Solution to be proposed.
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The wanted uniqueness is that of the LNodeType id, i.e. it is OK that both examples above fail (see LNodeTypeKey constraint). Further, the rule including lnClass checks that the lnClass an the LN instance and LNodeType definition are the same. The purpose of the iedType is just to show, that this LNodeType is reserved for a specific IED type, and may not be used by some other IED type.