4.9 A complete taxonomy of rules 87 4.9.1.3 Category transition constraints A category transition constraint specifies allowed or disallowed transitions between categories or statuses: for example, R79. A transition of the status of a customer from current to prospect is by definition impossible. 4.9.1.4 Complex concept structure rules A complex concept structure rule defines a particular constraint on one or more components of a complex concept. 4.9.1.4.1 Complex concept cardinality rules A complex concept cardinality rule defines the number of (or minimum and/or maximum number of) components of a particular type within a particular concept: for example, R80. A flight has by definition exactly one destination port.