Editing Solids and Surfaces - Autodesk AutoCAD 2013 Practical 3D Drafting and Design - page 198

Note
If there is no common volume, the command returns no object but erases the ori-
ginal objects.
If we want to create a volume corresponding to the intersection of objects but
want to maintain the originals, we should apply the INTERFERE command, which
will be presented in the next chapter.
Editing composite objects
Under some conditions, we may be able to modify original components of composite
objects.
If composite objects, obtained by unions, subtractions, or intersections, have recor-
ded history, that is, kept track of original components, we may edit them by modifying
these components.
To record history and have access to original components, we may select the solids
to combine and set History to Record on the PROPERTIES palette. By default, all
solids don't record history. If we want all solids to record history upon their creation,
we have to change the SOLIDHIST variable to 1 (by default, it is 0 ).
To select a component, we press the Ctrl key and click on a surface or edge caused
by the component we want to modify. Then we can use grips, the PROPERTIES
palette, and some editing commands, such as MOVE or 3DMOVE , to edit the compon-
ent. The composite object updates immediately.