The control has two properties to control what the Coordinate property returns and what coordinate is displayed in the text of the control.
bool - ProjectionPlaneTransform_Input
When true: If a this control has focus and the mouse is used to pick in a view with a UCS (UCSToWorld is not NULL) the result will be transformed properly. With CoordinateEdit, this will mean the field will be populated with UCS coordinates, not world coordinates.
if you plan to support the Cutting Plane View with this command, this should be set to TRUE in all cases unless you need to show world coordinates specifically.
bool - ProjectionPlaneTransform_Output
When true: When you ask the control for its stored data (Like with CoordinateEdit's Coordinate property) it will be transformed back to WCS coordinates. If false you'll get UCS coordinates/output.
Won't work when set to true while ProjectionPlaneTransform_Input is false.
There are also properties to get projection and world coordinates. .CoordinateProject will return the coordinate in the projected UCS and CoordinateWorld will always return world coordinates. These will be the same value if the coordinate wasn't defined/picked in a projected plane.
Yes, you have access to the plane definitions. The easiest way to find objects like these is to create a new project and use the existing TBC command to create the object in question. Then use the ViewProjectData macro to explorer the different objects defined in the project. You should see something like this;

So you can see the object that defines one of the projection planes is called "ProjectionPlane". Using Object Browser for the Trimble.Vce.Data assembly will show you the classes and their methods. You can see the static method ProvidePlaneCollection that you can use to get the collection that holds the list of projection planes. Then select the ProjectionPlane class in the Object Browser to examine the properties of the plane.
