The document was a CIA document, but classification came from State Department from what I read.
Here is the link:
https://www.cia.gov/library/publications/the-world-factbook/geos/sy.html
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“Kurdistan Workers Party (PKK) (Kongra-Gel):
aim(s): establish Kurdistan, which comprises territory in northern Syria
area(s) of operation: operational in the north combating ISIL, primarily in the Kurdish-populated region known as Rojava and Syrian Kurdistan; Salih MUSLIM Muhammad leads Kurdistan Workers Party’s Syrian wing, the Syrian Kurdish Democratic Union Party (PYD); majority of fighters inside Syria are Syrian Kurds, along with Kurds from Iran, Turkey, and Iraq”
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Issue is designation of PYD as arm of PKK. PYD is political portion of YPG.
The mention in the CIA World Factbook is not an official designation.
The World Factbook is an unclassified public document, provided for initial familiarization with an area. It is an informal informational resource, a step up in reliability from Wikipedia. Contractors compile and periodically review it for CIA.
Believe me, it does not commit the US Government to any policy position.
The State Department or the White House are the authoritative sources for official designation. Under certain circumstances, the Secretary of the Treasury can also make designations. If there was a formal designation, there is no way that we would miss it - it would be well covered in the US media. I would be shocked if Kurdish leaders were not told face to face ahead of time, if an official designation as a terrorist group was coming.
On the other hand, I doubt that the US military will ride to rescue of the Kurds in Afrin, guns blazing. I believe the simple fact is that they are outside of the US security perimeter. We have no open ground line of communication into Afrin, and the airspace has been previously agreed to be under Russian control.
I have no doubt that Kobane is under US protection, and based on recent statements I believe that Manbij is as well. In Afrin however, I expect only Diplomatic or covert support - not air cover.
Perhaps if atrocities continue to mount, International opinion will intensify against the Turks (this is slowly happening). Also, the US may find another issue, or another way to put pressure on the parties involved in conducting, or allowing, the attack on Afrin. I saw Heather Nauert (official State Department Spokesperson) mentioning a new suspected use of chemical weapons by the Syrian regime:
Previously, the US struck the Syrian regime quite hard over this issue, so this poses a serious (potentially existential) threat to the Syrian regime. This could be a veiled ultimatum to Assad. Or it might just be a coincidence.