When the Duct or Duct Placeholder tool is selected, the Modify | Place Duct tab provides the following options for placing duct:
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Justification: opens the Justification Settings dialog where you can specify Horizontal Justification, Horizontal Offset, and Vertical Justification for the duct. Note: This options is not available when the Duct Placeholder tool is selected. -
Automatically Connect: lets you automatically connect to the snaps on a component when starting or ending a duct segment. This option is useful for connecting segments at different elevations. However, when drawing duct along the same path as another duct at a different offset, clear Automatically Connect to avoid making an unintentional connection. 
Inherit Elevation: inherits the elevation of the element being snapped to. -
Inherit Size: inherits the size of the element being snapped to. 
Ignore Slope to Connect: controls whether sloped round duct connects using the current slope value or connects directly regardless of the slope value. Note: This option is only available when placing round duct.-
Tag on Placement: applies the default annotation tag to a duct segment when it is placed in the view.