Construction entities are used as a reference and do not create solid geometry. It is common to use construction lines in sketching to indicate that arcs or circles lie along the same line or to indicate the midpoint of a line. An example of a use for construction circles is shown here:
To sketch a construction entity, in the Format panel, click (Construction) and sketch an entity (e.g., line, arc, etc.). To toggle an already sketched entity from solid to construction (or vice versa), select the entity and click the icon again.
This tip is from the ASCENT training guide: Autodesk Inventor 2017 Advanced Part Modeling