Unload allows you to move loaded units from a transport into a neighboring tile. However, only the Marine, Storm Marine and Swarm can unload into enemy occupied tiles.
Great Library entry[]
Historically, ships and other vehicles have been critical for their use as transports, vehicles in which to move military and civilian units. In ancient times warriors sailed to battle in oar-driven Triremes. Over time the vehicles used to transport military forces became more and more advanced and included sailing vessels, troop ships, cargo planes, undersea crawlers, and space planes.