When creating an edge flange, sometimes you might want the new flange to be perpendicular or parallel to a specific face or plane.
This is very beneficial if you have created lofted bends as your sheet metal part.
When you create your new edge flange feature, there is an option to select a face to make the newly created flange perpendicular or parallel to.
This must be a face, you cannot select a plane at this time. However, you can extrude a surface that resides on a plane (as another feature before creating your new flange) and use that as a selectable face. That is the blue face that is in the picture above.
Once the face is selected, you can tell SolidWorks to make the new flange perpendicular or parallel to that face.
This is much easier than trying to figure out what angle will make the new flange parallel with one of your primary planes.