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As already discussed, at least one of the properties of a subject reifier must be a "subject identity property". (All properties are either subject identity properties or they aren't. If they aren't, they're called "other properties".)
A reifier can have any number of subject identity properties, but each one must be defined by a different Topic Map Application. In other words, when a topic map is viewed in such a way that only the properties of reifiers that were defined by a single given Topic Maps ontology are visible, every reifier has exactly one subject identity property. (A third way to say the same thing appears in the title of this slide.)
When an ontology defines a subject identity property class (in other words, when it declares a property name), it must also disclose the technique that should be used in order to determine whether two reifiers should be deemed to have the same subject, and should therefore be merged and become a single reifier, when they both have instances of that property class.