A single D card is required to give the date and time when the 
event artifact was created.  This is different from the time at which
the event occurs.

A single E card gives the time of the event (the point on the timeline
where the event is displayed) and a unique identifier for the event.
When there are multiple artifacts with the same event-id, the one with
the most recent D card is the only one used.  The event-id must be a
40-character lower-case hexadecimal string.

The option P card specifies a prior event with the same event-id from
which the current event is an edit.  The P card is a hint to the system
that it might be space efficient to store one event as a delta of the
other.

A single U card gives the name of the user to added the attachment.