In the latest update S3D added the possibility to add supports while in cross section view, which is potentially useful. However, it seems to me they implemented it quite badly.
If the structure you want to support is cut out by the section view, you can't add supports to it, just as if it wasn't even there. For the simplest example, imagine a hollow box where you want to support the roof internally. Make a section view in the Z direction to remove the roof from view, and try to add supports to the floor. Not possible, because S3D seems to think the roof is gone just because it is hidden from view.
In some cases you can of course work around this by arranging the section view differently, but sometimes that's inconvenient. Why on earth does S3D behave like this? A section view is just a visual thing, and S3D should of course function normally even if something is temporarily hidden.