When an ObjectType needs to expose several subcomponents of a specific type without defining the exact number or the BrowseName, the MandatoryPlaceholder (at least one) or the OptionalPlaceholder ModellingRules are used. Often, this might be combined with a grouping Object in order to easily browse all subcomponents of that type.

When an ObjectType requires at least one subcomponent of the specific type, the grouping Object must be Mandatory and the subcomponent of the specific type must use MandatoryPlaceholder.

When the ObjectType is not required to have at least one subcomponent, options are:

1. The grouping Object is Mandatory and the subcomponent uses OptionalPlaceholder
2. The grouping Object is Optional and the subcomponent uses MandatoryPlaceholder
3. The grouping Object is Optional and the subcomponent uses OptionalPlaceholder

When the grouping Object has additional functionality like a Method to add to its subcomponents it is recommended to make the grouping Object Mandatory and therefore the subcomponent has to use OptionalPlaceholder (Option 1).

When the grouping Object does not have any additional functionality like a Method to add to subcomponents, it is recommended to make the grouping Object optional so a client does not have to browse the empty grouping Object to figure out that it is empty. In this case, it makes sense to define the subcomponent as MandatoryPlaceholder so the grouping Object is only available when a subcomponent is included (Option 2).

However, when the model is built to be extended and a subtype may add additional functionality to the grouping Object, defining the subcomponent as OptionalPlaceholder allows subtypes to change the ModellingRule of the grouping Object without requiring a subcomponent of the specific type (Option 3).