Great input Loki - that helps a lot.
What if any virtue is there in having artillery at the Army or Army Group level? You've talked about loading up Inf-based Corps with it, so should I devolve artillery currently at the SHAEF, Army Group or Army level down to my inf Corps mainly, and have a sprinkling in each Arm Corps?
I would presume different Art units have different ranges - where do you see that if so? There are, I've really only just realised through this conversation, that there are dozens of Art battalions from 105's to 240's sitting attached to SHAEF in hence presumably in London...Do I urgently start leveling those out to the Divisions, Corps and/or Armies?
Same with the Engineering units - Construction Engineers and Combat Engineer battalions?
For that matter, what about the gazillions of AA units sitting against SHAEF somewhere presumably they aren't ever going to get a shot at passing bombers? Are they relevant in the French scenarios like Breakout and Pursuit?
I don't think there's any reason to have artillery battalions at army or higher level. Move them into your corps. Loki has some good advice on a Siege Corps with the heavy artillery.
Unfortunately you have to use the editor program to see the ranges and effects of different guns. I don't know why the game developers didn't include the ability to show range and effectiveness figures for different ground weapons.
NOTE that in at least some scenarios the game will swap out some artillery type for another, not necessarily to your advantage. I've seen 155mm guns replaced by 155mm howitzers, and 155mm howitzers replaced with 105mm howitzers. And this problem is compounded by the fact that the game will still list the art unit as being a 155mm unit even though it now has 105mm instead. See screenshot.
Construction engineers and combat engineers are VERY different. You assign the former directly to cities to repair Ports and Railways and/or help Airbase construction and expansion. The game will assign free ones to on-map repair of rail hexes, but you'll find that once your Allied turn starts, the repairs already are completed, and you can "Return to HQ" the unit or go to a nearby city screen and assign that unit to the city to do necessary repairs.
Combat engineers have CV and special abilities to reduce fortifications.
By the time of Breakout and Pursuit and against the AI, the allied AA units serve no purpose. I assign them to corps, more out of a subconscious urge for historical realism, but if you have the spare Admin Points, you should disband them so they go into the manpower pool. Depending on the scenario and your style of play, you may find the British, Free French, Polish, Czechs, Belgians, Dutch, Greeks, and/or Canadians running low on Manpower. Look at the Pools screen (press P), and make sure you switch to "Active Pools" in the very bottom right corner.