A structural relation between a state of an object or value of an attribute and another object or its state or value, meaning that these two entities are associated with the tag expressing the semantics of the association. In case of a null tag (i.e., the tag is not specified), the corresponding default null tag is used. Three groups of state-specified tagged structural relations exist: (1) source state-specified tagged structural relation, (2) destination state-specified tagged structural relation, (3) source-and-destination state-specified tagged structural relation. Each of these groups includes the appropriate unidirectional, bidirectional, and reciprocal tagged structural relation, giving rise to seven kinds of state-specified tagged structural relation link and corresponding OPL sentences. More information and examples can be found in Model-Based Systems Engineering with OPM and SysML, Chapter 3.3 "Adding structural links". Relationship cardinalities Object multiplicity in structural and procedural links Object multiplicity shall refer to a requirement or constraint specification on the quantity or count of object instances associated with a link. Unless a multiplicity specification is present, each end of a link shall specify only one thing instance. The syntax of an OPL sentence that includes an object with multiplicity shall include the object multiplicity preceding the object name, with the object name appearing in its plural form. Multiplicity specifications may appear in the following cases: to specify multiple source or destination object instances for a tagged structural link of any kind; to specify a participant object with multiple instances in an aggregation-participation link, where a different participation specification may be attached to each one of the parts of the whole; to specify an object with multiple instances in a procedural relation. Object multiplicity expressions and constraints Object multiplicity may include arithmetic expressions, which shall use the operator symbols "+", "–", "*", "/", "(", and ")" with their usual semantics and shall use the usual textual correspondence in the corresponding OPL sentences. An integer or an arithmetic expression may constrain object multiplicity. Graphically, expression constraints shall appear after a semicolon separating them from the expression that they constrain and shall use the equality/inequality symbols "=", "<", ">", "<=", and ">=", the curly braces "{" and "}" for enclosing set members, and the membership operator "in" (element of, ∈), all with their usual semantics. The corresponding OPL sentence shall place the constraint phrase in bold letters after the object to which the constraint applies in the form ", where