A relation or relational attribute is an attribute containing as values the ordinal number of the elements of another domain. It thereby defines a mapping from each element of the domain for which it is an attribute to that other domain-unit. The following picture illustrates this:
A relation has the source domain as domain unit and the related/target domain as values-unit. It’s a mapping from the source towards the elated/target domain.
It is advised to use the postfix _rel when naming index attributes, see also naming-conventions.