The Green Button Test Cases.docx document contains tests for the atom:ids, they must be UUIDs. An important aspect of these UUIDs not yet convered in the tests is that they must be stable, i.e., they must not change even if the same Green Button data is downloaded again at a later time.
For example, suppose I download my Green Button data today and it contains a MeterReading consisting of 5 IntervalBlocks (from January through May). Now suppose that I download my Green Button data again next month, then it contains the same MeterReading, this time consisting of 6 IntervalBlocks (from January to June). Then I would expect the MeterReading UUID to be the same in both downloads, and also the first 5 IntervalBlocks to have the same UUIDs in both downloads.