Underdefined, Fully Defined, and Overconstrained Sketches
The Sketch Legend and Model Browser icons indicate whether a sketch curve is underdefined, fully defined, or overconstrained.
Ideally, sketches should be fully defined before creating geometry from them.
        - A sketch entity is underdefined when its dimensions and/or position in the sketch plane have not been fully specified.
 - A sketch entity is overconstrained when one or more conflicting constraints have been applied.
 - Underdefined or overconstrained sketch entities may result in errors in downstream modeling operations.
 
Sketch Legend

Note: You can show or hide the Legend in the
                    Preferences under  or by right-clicking the workspace and selecting Show
                        Legend.
            Model Browser Icons
Underdefined
Fully Defined
Overconstrained