For Object and Variable the ModellingRule MandatoryPlaceholder has a similar intention as the ModellingRule OptionalPlaceholder. It exposes the information that a TypeDefinition expects of instances of the TypeDefinition to add instances defined by the InstanceDeclaration. However, MandatoryPlaceholder requires that at least one of those instances shall exist.
For example, when the DeviceType requires that at least one DeviceParameter shall exist without specifying the BrowseName for it, it uses MandatoryPlaceholder as shown in Figure 25. Device A is a valid instance as it has the required DeviceParameter. Device B is not valid as it uses the wrong ReferenceType to reference a DeviceParameter (Organizes instead of HasComponent) and Device C is not valid because it does not provide a DeviceParameter at all.
The ModellingRule MandatoryPlaceholder requires that each instance provides at least one instance with the TypeDefinition of the InstanceDeclaration or a subtype, and is referenced with the same ReferenceType or a subtype as the InstanceDeclaration. It does not require a specific BrowseName and thus cannot be used for the TranslateBrowsePathsToNodeIds Service (see OPC 10000-4).
When the InstanceDeclaration is complex, i.e. it references other InstanceDeclarations with hierarchical References, these InstanceDeclarations are not further considered for instantiating the TypeDefinition.
For Methods, the ModellingRule MandatoryPlaceholder is used to define the BrowseName where subtypes and instances provide more information. The Method definition with the MandatoryPlaceholder only defines the BrowseName. An instance or subtype defines the InputArguments and OutputArguments. A subtype shall also change the ModellingRule to Mandatory. The Method is mandatory for instances.