D — information about a change in a data value for a single variable
A D contains the information of a change in a data
value that occurred when the edit check named in the parent element was
executed.
It identifies which variable (V) was changed,
what the original value of the variable was (O), and
what the new value is (N).
The V element identifies the variable for which the
data value was changed.
In this context, it is not expected to have any child elements.
The O element will always be present, even if the
variable had no original value (was blank), and similarly the
N element will always be present, even if the variable
has no new value (becomes blank).