Is a transformer vector diagram based on a Primary-Secondary or High Voltage - Low Voltage relationship?

Transformers have their voltages arranged in many different vector arrangements. The vector description is based on an order determined by High Voltage - Low Voltage and not Primary - Secondary.

As as example, a common arrangement is delta to wye with a neutral.

A 480 to 208Y/120 step down transformer would use the vector description of Dyn1 (-30 degrees).

A 208 to 480Y/277 step up transformer would use the vector description YNd1 (-30 degrees).

A 480 to 480Y/277 isolation transformer would use the vector description Dyn1 (-30 degrees)