To offset a plane means to have the 3D plane object be above or below the points that define it. For example, a plane might be fit to five 3D points, but you need the final plane to be above these points – this would involve a positive plane offset. There are several reasons why you might want to offset a plane along its normal.
The plane offset is defined by a property. See Plane Properties and Preferences - Application Settings - 'Curves and Surfaces' section for a default plane offset.
A common use of plane offsetting is when you are using surface drawings or outlines on a plane but the points that define the plane are not at the height of what is being traced. One example of this is with rubber-backed Letters Sheets. The coded targets of these Letter Sheets are 4 mm higher than the surface they sit on – which is usually the surface being traced (like with a boat deck). In this case you want the plane offset to be -4 mm.
Another example is if you are using 2D digitizing methods with surface draw or outlines on a gasket or thick board that sits on a surface with thin paper targets. This means the targets (and the defined plane) are lower than the object being traced. Here the plane needs to be offset upwards. Offsetting the plane will improve the accuracy of the tracing.
These diagrams from the side shows the relationship between the planes and the objects being traced:
In the diagram above, the blue and green boxes are a camera looking down. Heights are exaggerated for illustrative purposes.
• In the left diagram, the top of the gray box is where you are tracing (like on the surface of a boat deck), and the top of the black box is where the coded targets are (like on the rubber-backed targets) – the Plane needs a negative offset equal to thickness of black box.
• In the right diagram, the top of the gray box is where the coded targets are (like paper targets on a floor), and the top of the orange box is where you are tracing (like on a wood template or a gasket) – the Plane needs a positive offset equal to thickness of orange box.