It must be a vertex object. Assuming you have two vertex objects in the Modelling Room that overlap: Ctrl + A to select the whole of both objects (everything is red). Now you can select Boolean Union or Boolean Intersection from the Polymesh menu. For Boolean Subtraction, with both vertex objects selected [everything red], Ctrl+Alt click on one point(vertex) of the object you want to remain. The point should turn yellow (for some reason it doesn't always on my machine but it still works). Then choose Boolean Subtraction from the Polymesh menu. HTH