I imported some conduits from model builder, and it has created automatically few water nodes with a 3D good looking.
I intend to move those created nodes (as shown in the picture below) to a library and make them able to manage with the feature definition option.
When you use tools like ModelBuilder, or Extract Utilities from Graphics, nodes can be automatically created at the ends of conduits. If so, the software needs to draw something for each node in the 2D and 3D models, and - if you haven't specified a node feature definition to use - then default graphics are created.
If you want to create a feature definition for a node that will create the same default graphic, then do not select a cell in the element template. When the node feature definition is used, the software will detect that no cell has been defined, and it will create the graphic automatically.
You probably only want to do this in the element template for the solid feature symbology, as you will probably have a cell that you want to use in the element template for the plan feature symbology.