When you develop geometry in the formed (bent) state you can project datum curves to communicate information from the bent state to the flat state. You sketch and project a curve onto the surface of the sheet metal part. You place the curve by either following a surface when the model is bent or unbent, or by following a surface during a bend back operation (if the part is in the unbent state).

See the Part Modeling module for more information about working with projected datum curves.