Think the term 'Lightweight' is referring to how the section view is created as opposed to the contents of the view. For one thing, you will note that the 'Lightweight Section View' looks and acts very much like the Dynamic 'View Section' that can be used to quickly see the interior of a model or assembly. Ans that's because it's using many of the same bits of technology to get the view that you see, unlike the older PMI 'Section View' function which required that you define a sectioning or 'cutting' body that then used a Boolean Subtract to get the final model. Now both methods are still supported since the 'Lighweight' section is limited to just a single full cutting plane, while the 'Section View' function can create such conditions as quarter-sections. or to remove a pie-shaped or wedge section from a rotational part or assembly. The reality is that to make the more general 'Section View' more work must be done and thus is could be considered a 'heavyweight' function. Of course we would never call it that directly, but naming the new 'Lieghtweight Section' as we've done, it has indirectly implied that the other method is indeed 'heavyweight'.
So what this all means is that if you're only interested in a simple single-cut sort of 'Section' view, use the new 'Lightweight' function, but if you need partial sections or special shapes, use the traditional 'Section View' function.
For the visualization of their difference,
look at this pictures,
Example I have this Assembly Model.
now this is my REGULAR section view:
this typical section view cuts all the objects that touches the plane, next is the LIGHTWEIGHT:
As you can see, you can just select the object to be cut off, for my example, i just selected the upper case of my assembly and that is the only thing that is cut with in the plane though the plane touches the other objects.
If you want to clip a section of the whole model in the graphics area, you can use the typical section view. If you want to section only some of the objects like my example, use lightweight section view.