When you start the PartEditor, the GeometryTree replaces the KinematicsTree.
This is the symbol at the head of the GeometryTree  labelled 'ConstraintLoops'. 
Example: Geometry in ElementExplorer 
What is a ConstraintLoop? Each ConstraintLoop [labelled as 'Constraints'] lists the constraints that you have added to or between sketchelements. The Points are also in the Constraint Loop. Constraints that relate a set of sketchelements are listed in one ConstraintLoop. Constraints that relate a different set of sketchelements are in a different ConstraintLoop. If a Constraint relates the two different sets of sketchelements, the two ConstraintLoops combine into one ConstraintLoop that lists all of the Constraints. For example:

You can understand how the sketchelements in your model are constrained to each other with existing constraints and if they are needed by other elements in the MechanismEditor.
If you click a sketchelement in the graphicarea, then constraints you have applied to it:
•  become 'selected' in the GeometryTree 
•  show in the SelectionWindow 
If you click a constraint in the GeometryTree, then sketchelements you have applied to it:
•  become 'selected' in the GeometryTree 
•  show in the SelectionWindow 
•  also, shown in the SelectionWindow, are the elements that need the sketchelement in the MechanismEditor. 
Example: Geometry in ElementExplorer 
What is a ConstraintLoop? Each ConstraintLoop [labelled as 'Constraints'] lists the constraints that you have added to or between sketchelements. The Points are also in the Constraint Loop. Constraints that relate a set of sketchelements are listed in one ConstraintLoop. Constraints that relate a different set of sketchelements are in a different ConstraintLoop. If a Constraint relates the two different sets of sketchelements, the two ConstraintLoops combine into one ConstraintLoop that lists all of the Constraints. For example:
