All elements that make up a vessel exist between nodes. On the top part of the General Input tab are two fields that indicate From Node and To Node:
Every main element (not attachments such as nozzles) has two nodes. Consider these two vessels shown below:
Notice that every element from the bottom to the top or from the left to the right exists between two node numbers. There is a From Node, and there is a To Node. In the case of the bottom (or left) head, the nodes are situated at the tangent line and the weld line.