Do one of the following:
With a 2D line selected, click the Projection Line button 
on the command bar.
Right-click a 2D line and choose Properties. On the Format page of the Element Properties dialog box, set the Projection Line option.
On either the Line command bar or the Format tab of the Element Properties dialog box, select a projection line type from the Projection Line Type list.
Tip:
You also can create a line as a projection line. Select the Projection Line button on the command bar before you draw the line.
Dimensions and annotations connect to the defining segment of the projection line (the original 2D line on which the projection line is based).