Space Units and Land Units. Space units are cruisers, fighters, bombers and fleet commanders. Land units are tanks, soldiers, plex soldiers or artillery. Some heroes can be used in both land and space. Sending land units into a space battle will result into all your land units becoming medium transports, wich in games like X-Wing Alliance is pretty strong, but even the weakest TIE Scout squad can take one down in Empire at War. there are also special transports, like for Obi Wan Kenobi in Empire at War. it is slightly stronger, but still is pretty weak. Han Solo also uses a transport, but his transport is actually meant to be in space. his YT-1300 Corellian freigter is a really strong unit in space. Sending space units into a land battle is impossible.