Windows Phone 8.1 is still in the early stages of rolling out to devices in over the air updates, but Microsoft is hard at work at future updates, including general distribution releases or GDRs. Those GDR updates, usually referenced just as Update 1 or Update 2, bring patches, minor fixes, a few new features and enable additional hardware options for manufacturers (OEMs).
Evidently, references to GDR1 are found on the Windows Phone dev center under the OEM documentation. Unfortunately, if you try to click the links you are asked to log in, making this are restricted to just OEM partners.
Full story from the WPCentral blog...