Constraints are evaluated at the object level (so, after the 'XForm node).
This means that they can modify object parameters (like position) after the
animation of these node has been evaluated. A constraint is always linked to a driven
object node and most constraints also need to be linked to a driver object node.
All Constraints have an 'Influence' parameter. This parameter allows to control
how much the constraint is influencing (or supersede) the animation.
Constraints can be added in two ways:
Using the 'Add' combo-box in the parameter area;
Using the 'Constraints...' tool.
The constraints are always declared in the driven object node.
When a driver node is needed, the constraint node has a driver parameter. The parameter
can be set by pushing the '..' button:
Select the node type in the top combo-box, then double-click on the node name in the list
(or select a node and press the 'Ok' button.
The constraints parameters are available in the parameter area. All constraints have a top darker row
displaying the constraint type and four action icons: 
.
The first and second icons allow changing the priority of the constraint. Top listed constraint
are evaluated before bottom listed constraints (so, if two constraints influence the same attribute,
the last one will set the final value of the attribute).
The third icon allows showing/hiding the constraint parameter. The last icon deletes the constraint.
The available constraints are:
Position: The position of the driver object is copied to the driven object;
Rotation: The rotation of the driver object is copied to the driven object;
Scale: The scale of the driver object is copied to the driven object;
Limits: The position, rotation and/or scale of the driven object are limited to some range;
Point At: The rotation of the driven object are set so that one axis the driven object is
pointing at the pivot of the driver object;
Hinge: The driven object is rotated around one axis to point at the driver object.
Position Constraint
The position constraint copies the position of the driver object to the driven object.
If 'Influence' is '0', then the position constraint has no influence on the driven object.
If 'Influence' is '1', then the position constraint is setting the position of the driven object.
Between the two values, the position of the driven object is blended.
The 'Delta' parameter offsets the position of the driven object by adding a fix amound to
the position of the driver object. The 3 checkboxes allow to remove the constraint
influence on uncheck axes. If all 3 checkboxes are uncheck, the constraint has no more
influence on the driven object.
Rotation Constraint
The rotation constraint copies the rotation of the driver object to the driven object.
If 'Influence' is '0', then the rotation constraint has no influence on the driven object.
If 'Influence' is '1', then the rotation constraint is setting the rotation of the driven object.
Between the two values, the rotation of the driven object is blended.
The 'Delta' parameter offsets the rotation of the driven object by adding a fix amound to
the rotation of the driver object. The 3 checkboxes allow to remove the constraint
influence on uncheck axes. If all 3 checkboxes are uncheck, the constraint has no more
influence on the driven object.
Scale Constraint
The scale constraint copies the scale of the driver object to the driven object.
If 'Influence' is '0', then the scale constraint has no influence on the driven object.
If 'Influence' is '1', then the scale constraint is setting the scale of the driven object.
Between the two values, the scale of the driven object is blended.
The 'Delta' parameter offsets the scale of the driven object by adding a fix amound to
the scale of the driver object. The 3 checkboxes allow to remove the constraint
influence on uncheck axes. If all 3 checkboxes are uncheck, the constraint has no more
influence on the driven object.
Limits Constraint
The limits constraint restrict some of the 'XForm' parameter to user define ranges.
Each parameter have a checkbox telling if the constraint is restricting this parameter and a range.
Point At Constraint
The point at constraint is setting the rotation of the driven object so that the local axis defined
in the combo-box is pointing to the pivot point of the driver object.
The point at constraint also allows to define the driven up axis. This axis must be different than the
pointing axis otherwise the constraint evaluation might leads to unexpected values.
Hinge Constraint
The Hinge constraint rotate the driven object around a defined axis so that the driven object is pointing
(as much as possible) to the pivot point of the driver object.
Such constraint can be used to create billboard (an alpha textutred plan rotated to point to the camera).