3D SELECTION BOX IN REVIT 2015 AND EARLIER VERSIONS

Truth is I didn't know about this. Eventhough Revit 2016 has a specific tool for this, we can emulate it in 2015 and earlier versions by creating a 3d selection box in the following manner:

Suppose we want to isolate and do further work in the stairs of this model. First we need to select and filter to leave only the necessary elements:

Elements are isolated with the small glasses (Isolate Element), and in the view's properties the "Section Box" is checked.

See that? It generates a selection box which isolates only the visible elements. Going back to the complete view, apply the "Reset Temporary Hide/Isolate" command and you get a cropped model ready to let us work or show only that isolated portion.