Selection Shape:

The set of geometric elements that the user selects to communicate intent. This is different from the contents of the SelectionObject. The selectionObject contains the full selection as FreeCAD understands it and here we're only referring to the elements that the user intentionally selected.

Target Shape:

Using the user selected elements and other logic, Path derives a 'Target shape'. This is the input shape to the Path generators. Target shapes have several subtypes.

Spot Shape

Spot shapes are single axis actions like Drilling, Boring, helical clearing, tapping, etc. The are represented as cylinder where the central axis represents the center point of the operation as well as the start and end points. The diameter of the cylinder indicates the maximum area to remove. A Spot shape might be drilled with a smaller bit but not a bit larger than the diameter of the cylinder, for example

Line-Following

Profile-like operations take as input a line-following target shape. This is represented as one or more faces. The tool orientation is parallel to the face (perpendicular to the face normal). The faces may be sliced horizontally to indicate the steps down as well as the edges (top and bottom) to follow.

Area Removal

Area removal indicates a volume to removed via pocket/clearing type operations.

Face Decoration

This is similar to an area removal shape but has no depth. It is also like a line-following shape (one or more faces) but tool travel is parallel to the face normal. Face decoration is used as input for engraving/hatching/jewelling style operations.

Removal Shape

If it is possible to determine the actual material removed by an operation, the representation of that material is called the 'RemovalShape' The removalShape differs from the TargetShape by not including material that is unreachable by the tool.

REST shape

The 'REST' shape is the inverse of the RemovalShape. It is the material left over. When calculated, the REST shape becomes the input or Target for a subsequent operation.

NO-GO Shape.

This indicates a region that should not be traversed by the tool. It may indicate a clamp, vise or other obstruction in the machine's working envelope.