A <Part> is a group of geometrical elements that defines the space to check for gouges. A part is always to consist of surfaces. Those may be either specified explicitly or generated from curves by some way. For example, it is possible to define a revolution surface by its generating curve or a prism by its base curve.
The part is usually specified in the root node of the machining tree. Thus all the operations use the same part by default since the default part of an operation is a reference to the part of the preceding operation. Optionally one can alter this rule by overriding the default part of an actual operation.
The surfaces of the part are used to eliminate undesirable part gouges while the surfaces to be machined are specified in the <Job Assignment> node.