EN 15907 Relationships
Definition from the standard
A relationship associates an instance of an entity with another instance of an entity. For each relationship defined in this standard, a cardinality of zero or more is assumed for both ends, resulting in a many-to-many association. This may be restricted to a one-to-many association in suitable cases (e.g. for HasVariant).
Relationships can be implemented in many ways, depending on the purpose, the modelling paradigm, or architectural constraints of the chosen platform. Therefore, this standard does neither specify any form of expression for relationships, nor how entity instances are identified and referenced.