What's a MOC?
MOC stands for My Own Creation. It can be build only digitally. Or it’s a modification of an original LEGO® model (for example: you make a RC-version of a non motorized set). Or you throw all your LEGO® blocks in one box and create something completely new!
So it can be whatever you build! An alternate is also a MOC, but it’s made with pieces from just one LEGO® set. If you own the set, you can build the alternate.