The BPEL Designer diagram provides placeholders or “drop-zones” on the diagram that prompt you where to insert an element. These drop zones enforce the rules of the WS-BPEL 2.0 specification for the BPEL Designer.
When you drag an element from the Palette to the design canvas, the drop-zones appear to show the acceptable drop points for the element. These drop points reflect the construction logic of the diagram. Align the dragged element with one of the drop-zones and release the mouse button to insert the element.