The conditional expression that defines a relationship. The specific association between objects.
Any conditional expression
Objects mentioned in the RelCondition should exist before the relationship is created. Referenced objects that do not exist when a RelCondition is created are interpreted as character strings if SET QUOTING is OFF.
Employees.DeptNum = Departments.DeptNum
defines a relationship between employees (in entity set Employees) and their departments (in entity set Departments) using a common department number (DeptNum).