Upvote:8
Valid for it has mentioned as AT which stands for Austria according to my understanding.
Yes, that is correct.
Does that mean I need to enter schengen territory from Austria.
No, a D-Visa may also be used as a C-Visa for the other member states. This was, in the past, entered as a D+C but since most were issued that way it was simplified.
Transit was always allowed in the past (when there as no +C), but now you can stay as a normal visitor on your way the the issuing country.
The SCHENGEN STAATEN term is only used for C-Visa's.