According to the BPMN2 spec section 12.2:

All BPMNShape [and BPMNEdge] elements are owned directly by a BPMNPlane (that is the root element in a BPMNDiagram), i.e., shapes are not nested within each other in the BPMN DI model although they may appear that way when depicted. The bounds of a BPMNShape are always relative to that plane's origin point and are REQUIRED to be positive coordinates. Note that the bounds' x and y coordinates are the position of the upper left corner of the shape (relative to the upper left corner of the plane).