CalculatedRelations are virtual cross-references or "lookups" that don't have to be defined in advance (using indexes, arrays, etc.) The idea is that one can use just about any *expression* to "join" two or more different datasets without having to plan for such relationships in advance.

It is allegedly a prime feature of RelationalDatabases.

An example would be two tables of two unrelated events (Say, Phone_Sales and Web_Views).  You might want to generate sums and counts grouped by hour and join them together in one Hourly_Statistics table for a report or just to look for some hour-based correlation, even though joining the two original tables by their date and time columns would make no sense.
-----
See Also: ModernDinosaur, NetworkDatabase, OoLacksMathArgument, ImperativeVsDeclarative?
----
CategoryRelationalDatabase