Outdoor wire that is grey in color can be installed outdoors underground without conduit.
The grey outdoor electrical wire also known as direct burial romex can installed underground without being put into conduit.
The outdoor romex wire can also be instaleld outdoors without conduit such as on walls, poles etc.
However if installed outside I would recommend installing the outdoor wire inside conduit as from my experience of having outdoor grey romex directly in the sunlight has caused the insulation to harden up and it turned a brown color.
Then the insulation can degrade and come apart.
So It's best to run the outdoor wire inside some plastic or metal conduit to protect it.
It also makes it less prone to copper theft as well.