Optional indicates that an actual parameter may not be specified for this formal parameter. The external function is called with a zero value of the specified type, which is false for Boolean, 0 for integer numbers, 0.0 for floating point numbers, NULL for string types, and an empty list of unspecified type, which must not be modified, for lists.
dll "some.dll" "DoOptional" function DoOptional (nParam1 : out number optional, nParam2 : in number optional); dcltrans transaction TMain begin // both actual parameters can be omitted DoOptional(); end TMain;