common import warning question

I get this a lot since I am somewhat confused about doing full imports or exports as sys or system.

When doing an import - >
- Warning - objects were exported by sys, not by you... Why the warning? What could happen?

Could anyone help me fill out this little matrix?

Export done as       Import Done as            Ramifications?
sys		    sys
sys                          system 
system                    sys
system                    system