The Rotate Label to Angle 0 command is used to rotate 2D ductwork labels. If you manually rotate the label without using this command, the label will be rotated back to match the duct angle when the 2D ductwork is redrawn.
To permanently rotate a duct label, go to
Ribbon: DM HVAC->Ducts-> 
Rotate Label to Angle 0
Pulldown Menu: DM HVAC->2D Drafting->Rotate Label to Angle 0
You will be prompted to select the duct label to be rotated.
Select duct or label to rotate:
The duct label will be rotated to a 0-degree angle. The label will remain rotated when the 2D ductwork is redrawn.