Using 'Slots' to annotate metadata

ISO/IEC 11179-6 Part 3, Section 7.2.2.4 The things Slot instances provide a dynamic way to add arbitrary attributes to instances of on Identified_Item (7.2.2.1). A Slot instance is associated with exactly one Identified_Item instance, through the association item_slot (7.2.3.3). An Identified_Item instance may be associated with zero, one or more Slot instances. All Slot instances associated with a particular Identified_Item instance must have a distinct name to allow each Slot instance to be unambiguously identified.