PartEditor  Constraints Menu and Toolbar
In the PartEditor, use the commands in the:
•  : to add sketchelements. This vertical and to the left of the graphicarea. 
•  : to add and edit constraints to and between sketchelements. This is vertical and to the right of the graphicarea. 
PartEditor: Constraints menu
PartEditor: Constraints Toolbar
In the PartEditor, the is to the RIGHT of the graphicarea.
MERGEPOINTS : Deletes a Point. You cannot remove a 'Merged Point'.
•  Use to join sketchelements endtoend, and remove one of the Points. 
COINCIDENT CONSTRAINT : Does not delete a Point. You can delete a CoincidentConstraint later and move the Points apart again.
•  Use to move together two Points, or a Point to a Line, or a Point to an Arc or Circle. 
•  Use for 'construction' geometry. 

You cannot overconstrain a sketchelement in the PartEditor [SolidWorks will allow you, but you are given a warning].
For example:
A Point has 2 degreesoffreedom [DOF]. It you add a 'coincident constraint' between a Point and a Line, you remove 1 DOF, and 1 DOF remains. The Point can slide along the Line.
You cannot then add a new constraint between the same Point and a Point at one end of the Line. A constraint between a Point and a Point would remove 2 DOF, but there is only 1 DOF that remains.

1.  Click a sketchelement in the: 
 or 
The sketchelement and the constraint show in the SelectionWindow.
2.  Rightclick the constraint in the SelectionWindow 
3.  Click 'Delete' in the contextualmenu 
