How to modify the reference boundary by dragging the handles
When working in some files, the reference boundary can be modified by dragging handles at the corner of the boundary but in other files, the handles do not appear at all. Modifying the boundary changes what elements are visible from the attached file.
Most likely, the files where the reference boundary handles can be dragged, are DWG files opened in MicroStation, or, references that have been clipped. If there are no handles available to modify boundary;
See the short video showing this: