This string dot variable represents a special null value meant to represent "nothing", and is separate from the empty string "". If printed, it typically appears as "<null>", but it should be referenced exclusively by the dot variable .NULL, or the dot function .ISNULL(var). It has a variety of particular uses, including:

•   An attempt to access an ordered map pair by its key will return .NULL if there is no such element. See Ordered Maps: Accessing Elements.

•   An existing ordered map pair may be deleted by assigning its value to .NULL,  whereas assigning it to the empty string "" merely changes the value of the item without deleting it. See Ordered Maps: Accessing Elements.

•   Similarly, an MLIST element may be deleted by assigning it the value .NULL. See Deleting MLIST Elements.

•   It is also used with the MLIST sublist to indicate a non-existent sublist, or to delete one. See Multi-level List Operations.

•   When passing complex types (collections, iterators, etc.) to functions, .NULL may be used to indicate that nothing was passed.

•   Sequential files opened with the filename .NULL have certain special properties. See .NULL Files.