In vanilla Advance Wars, units placed on a map without a corresponding HQ to go with them become "dummy" neutral units that still exist on the map but will not move as they have no AI assigned to them. In AWBW, this works differently; instead of becoming "dummy" neutral units like on the carts, they will disappear when the game starts, but also produce side effects that impact gameplay on that map. These units are referred to as 'ghost units'.
Any tiles occupied by ghost units cannot have units "created" on them, regardless of source. If the tile is a production facility (namely, a Base, Port or Airport), that property will be prevented from building units, even if it later changes hands to a different player. Attempting to highlight such a 'ghosted' property will change the cursor to a 'Prohibited' sign as shown in the image. Ghosted cities will also not produce/generate units for the CO powers of Hachi and Sensei.
Aside from unit production, the tile will behave normally, including granting income and healing/refueling friendly units in the case of properties.
While this may seem bad initially, this can be used by mapmakers to balance Hachi/Sensei specifically for a map, or use these tiles for other purposes than production of units. For example, ghost Ports and Airports will function similar to temporary (air)ports from Days of Ruin in that they provide a place to heal and refuel the corresponding units without allowing players to use these sites to deploy more units. Similarly, a ghost Port could be used as a bridge that both land and sea units can traverse, without offering the potential to be a production facility.