About the construction plane and reference planes
Tools in Alias place objects in an XYZ coordinate system. Normally this is the world space coordinate system, the absolute frame of reference for your scene. However, there will be times when you want to align objects where the orientation, position and rotation are different from the world space axes and origin.
The construction plane defines a temporary coordinate space that can be moved or rotated away from the absolute world space. You can also create construction planes that you position and rotate in relation to a curve or surface.
When the construction plane is active, the points you click or coordinates you type use the coordinate system of the construction plane, instead of world space.
There can only be one construction plane in a scene (that is, the current construction plane). All other planes are referred to as reference planes. You use the Plane tool to create a reference plane which you can use as input to tools requiring a plane, or as the construction plane.
You can convert a reference plane to a construction plane (and vice versa) at any time. You can also use reference planes with certain tools (Curve Planarize tool, Curve Section tool, and the Cross-Sections button in the Control Panel).
Example
For example, consider the following curve. We want to create a surface perpendicular to the curve at its end.
In world space this would be difficult: we would have to draw construction curves for the new surface, then try to rotate the construction curves to be perpendicular to the tangent at the end of the curve.
With construction planes, the task is much easier. With curve snapping 
on, we snap the new construction plane to the curve and slide it to the end.
Now we can draw the curves for the new surface much more easily. The grid is perpendicular to the end of the original curve, and the origin of the new coordinate system is the endpoint of the original curve.
Finally we use the Toggle Construction Plane tool to return to the world space coordinate system. The new construction curve is exactly perpendicular to the endpoint of the original curve. Now we can use Set planar or other surface building tool.
