2.3 Facet Application
Facets apply to the nominal type referenced in the element where the facet is declared. In the following example, the Nullable facet applies to the DateTime referenced type.
<Property Name="SuggestedTimes" Type="Collection(DateTime)" Nullable="true" />
In the following example, the Nullable facet can only be placed on the child element that references the DateTime type. Facets cannot be applied to Collection type references.
<ReturnType> <CollectionType TypeRef="DateTime" Nullable="true" /> </ReturnType>