In the Green Button Test Cases.docx document, test # FDFTP6 says: Verify that all "related" atom link hrefs match the "self" link of another entry.
But "related" atom links need not always match a "self" link of an entry. They may also point to a related collection, e.g., a UsagePoint entry contains a "related" link to a collection of MeterReadings, and this "related" link matches the "up" links of the individual MeterReadings. (Each MeterReading has a different "self" link, but they all have the same "up" link.)
This affects also test # D.17, which is about the "up" links of MeterReadings and says: Verify that a valid value for the data element in the Test Purpose / Requirement (feed/entry[*/*:MeterReading]/link[@rel="up"]/@href) is present.
It follows from the above that the valid value of this @href is a "feed URL" for the collection of all MeterReadings, it does not match the "self" link of any other entry.