This application works the same way for both assembly and part modeling environments. A single button has multiple uses:
1. Just press the button and get a section view through one of the origin workplanes. The workplane is automatically selected depending on how you look at the model.
2. Press the same button to end the section view.
3. Preselect part or assembly occurrence and then press the button to get a section view through one of the occurrence origin workplanes.
4. Preselect circular or conical face (circular edge) to get a section view through the axis.
5. Preselect a straight edge to get a section view perpendicular to the edge through the center point.
6. Preselect a workplane or planar face to get a normal half-section view.
Without preselection, the app works only with origin workplanes which are selected automatically! It is possible that the section gives a blank result in blank because nothing is left behind the selected plane. If so just run the app again to restore the full view. Then look at the model from a different angle so another section plane is selected.