Parts are holes within a polygon object. This function would typically be used to exclude a tree cluster from a paddock. To draw a hole:
•Select the layer the object is on.
•Click on the object to make active (border will be highlighted).
•Click on the Edit Drawing button 
•Right mouse click on the first point of drawing of the hole.
•Select New Part from the menu.
•Continue to draw around the area you want to make a hole.
•When finished, either double left mouse click when making the last point or click the Stop Drawing button 
The area of the hole will be deducted from the object size.
