The calculated data link is a special kind of data link. Instead of holding a static data value or connecting to a database as the simple data link does, the calculated data link is an equation represented by a network of nodes. It is similar to a dependency network in that it is constructed the same way, however, unlike the dependency network it can be (and usually is) constructed of non-logical nodes (arithmetic, trigonometric, etc.).
Its equation must have valid inputs as defined by its nodes. Otherwise the equation will evaluate to an ERROR.
The calculated data link simply returns the value of its equation. When the equation evaluates to ERROR (because of invalid or missing inputs) it is the same as having no data when it is being compared to an argument.