Physical craft : U.F.O.s, or "flying saucers", that operate in the same physical world as we do. They can be seen, touched etc. just like normal physical objects.
Non-physical craft : Some craft are from planets/times/dimensions of a different vibrational frequency to us. Effectively this means that they are generally (physically) invisible to us.
Physical/Non-physical craft : These are craft that can alternate between being in the same physical dimension as us and not being so.
Many craft prefer not to be seen by normal
earth people so chose to operate in non-physical dimensions. This is partly
to improve efficiency but also because many aliens are aware that they
shouldn't interfere in the spiritual progression of the people of this
planet at this time. This is in the general sense. There are people who
are currently working with aliens in anticipation of when things will change
in the future.