The 'Use Period Covering' control found in the source object type part of the Relation tab enables (or disables) linking objects whose history is contained in several rows of the database table.

In a period covering relation, the primary key of each object consists of 2 attributes (or 3 attributes if it is further identified by the Client field). One of the two attributes is an instance identifier (a string representing the identity of the object itself), the other is a period attribute (a string representing the interval of validity of the particular line). It is not possible to reference such objects by means of a standard foreign key. Instead, a combination of an attribute identifying the whole group of lines (instance identifier) with the period identifier compared to the current time is used.

Note: The term Client mentioned above is used for a region dividing objects in the database. The ability to support Client allows a company to create several completely independent and separate regions within one Valuemation database. Each client then has his own data and configuration and can work independently of the other clients. Maintaining separate regions of data is an efficient and secure way of using one installation to provide services for different clients. Users can switch clients both on logon and in the runtime during their work. The process of assigning users to clients is called client mapping.