How do you create Thiessen polygons for certain nodes in your model? For example there may specific nodes that can have a demand/load, while others should not.
Create a selection set or select the nodes in drawing that will need Thiessen polygons. In the Thiessen polygon window choose "Current Selection" and click the "Next" button. Proceed through the rest of the steps in the Thiessen polygon tool. When using Loadbuilder to import demands or loads using a method that utilizes service areas, they will only be assigned to the nodes within your selection set.