Artillery, Anti Aircraft and some other unit types are marked as stand off units. These are units that would not normally engage an enemy directly. These units will not enter the zone of control of an enemy unit if they know it is there. Also, these units cannot be attacked by other land units if protected by non-stand off units. This would mean that a stack of artillery attacked by an enemy could be destroyed easily but can be protected by a single infantry, tank or other such unit in the same location. Units cannot protect stand off units if they are supressed, out of ammo, retreating or are themselves stand off units.
Cargo boats with no attack values will generally be stand off units and as such will be unable to enter a pier/port that is guarded for unloading forces. These boats are intended for troop movements in secure areas.