Currently the Cause of Death Part 1 interval is specified as a 20 character string.
Some EDRS systems capture this as a Quantity (e.g., units=months, value = 3).
In order to support more precise exchange, and to align between VRDR and MDI, the option of providing a string or a Quantity is needed.

See https://build.fhir.org/ig/HL7/vrdr/branches/intervalasquantity/StructureDefinition-vrdr-cause-of-death-part1.html