I'm going to have to agree with the concerns around the static unit type.
From a pure military sense, a static unit is a non-mobile military asset. Static, by regular definition, also means something that doesnt move.
Static weapon positions are defensive military assets. They are used to defend high value positions, or interdict enemy forces moving through or into an area.
The towed anti-tank guns are a good example of mobile/static differences. In WW2, the anti-tank guns of all belligerents were used as defense assets by infantry formations to secure an area. They were towed into position, and early examples were light enough to have limited mobility to be move short distances by their integral crew. Later examples were much heavier (2 to 3 tons) and generally were field emplaced.
So, anti-tank guns were NOT mobile weapons in any strategic sense, and had little to no tactical mobility.
So, what does that mean in game? They dont move. They dont get used on the offense.
This can be anything from an infantry bunker to a naval gun emplacement.
The big difference would be on the level of fortification as Steve has already laid out. So, a regular towed AT-Gun could be fortified up to level 3 by the crews. That would range from basic field positions to dug in improvised bunkers. Anything past that would be combat engineered defenses, involving concrete.
The Maginot line for example, is a large static emplaced command. It ranged from individual anti-infantry machine gun emplacements to large anti-tank and field artillery bunkers. The big thing with these kinds of static positions is that they are offensively USELESS.
While the Maginot Line was very powerful, anything outside of the beaten zone of its guns was completely safe from the Line.
That kills a couple of birds with one stone. They are great for defense purposes, but dont fire on the offense.
Mobile systems are always a threat, because they can maneuver to bring their firepower to bear and engage enemy formations. Static positions CANT they can only engage an enemy that comes within its zone of defenses.
Historically, and even today, static positions are used because they are cheap, and very effective for their costs, for defensive purposes. It doesnt mean that they arent effective. The Soviet Union fielded entire anti-tank artillery regiments in WW2 to great effect, notable at the Battle of Kursk. But when the Soviets counterattacked at Kursk, it was with tanks, not AT guns. Historically, where AT guns showed great performance in the field, they generally got converted into assault gun/tank destroyer configurations to make them offensive weapons. They were cheaper than tanks, had had a larger gun to weight ratio than tanks, but were tactically less flexible.
Finally, their were examples of very large mobile filed guns, such as the famous railway guns used by the Germans in WW1 and WW2, but examples were used by all belligerents. Railway guns were considered MOBILE for their time, as infantry was all leg infantry (slow) and regular artillery was horse drawn. A direct TN Aurora example would easily be a railgun or laser mount on a large vehicle capable of keeping up with the rest of of the maneuvering forces.
Obviously things can get pretty blurry when you consider the range of weapons in question, but the short list is that static positions regardless of type, were cheap and effective defensive positions.
Mobile systems are designed to project offensive power against an enemy force, and are used that way. They are part of the maneuver element of a ground force, and are part of its offensive potential.
edit: Sorry this got a bit longer winded than I intended, but I did want to chime in about why statics shouldn't be firing on the offense and why.
