Containers are collections of resources. They are modeled by an instance of one of the three subclasses of rdfs:Container: rdf:Bag (an unordered collection), rdf:Seq (an ordered collection) or rdf:Alt (an alternative). Membership is modeled by automatically generated properties rdf:_ 1, rdf:_ 2, etc. These properties are all instances of rdfs:ContainerMembershipProperty, a subclass of rdf:Property5.
This mechanism for modeling collections of resources is much controversed among RDF implementors: the numbering of membership properties is contentious especially when neither rdf:Bag nor rdf:Alt need an order relation between their members. Furthermore, these properties can not be expressed in any schema since there is an unbounded number of them. This can even cause infinite loops in agents using the XML syntax (see annex A).