There are three categories.
- Artillery (rarely seen, bugged)
- Engineers (seem to work - 2% increments)
- Combat Intensity (seem to work - whole level reduction)
This third one shows the size of the attacker, so indicates Battle/Combat Intensity. That's the one many, including me, assumed to be/include Artillery. But it doesn't.
The cause for the defenders Fort Reduction is not as the manual states the (attackers) artillery/heavy artillery, but the defenders rockets.
Not sure if there are other elements bugging it, but rockets always seem to work.
So that's the Bug I want to report. It seems consistent, once there are no Rockets it works. While I could not get the Fort redced whatsoever without any Rocket Ground elements present. Not exacly sure if it is related to Ground element types ...or whatever.
Here I have tests with Ground Element type: 84 - "Unarmored SP Rocket" and type 31 - "Rocket". I did test the type 56 "SP Rocket Launcher" in the Editor, also seem to have worked (got no Combat Log). So it is about the Rockets of all kinds. Attached an Example Battle. It's vanilla VtB, just move the 2 Rocket Brigades to the frontline near the Kurland Pocket and additionally assign all available Rocket SUs to the Army HQs of them/the Units in their Hex. Then attack in German turn. This also works only with the SUs (that's how I got the "14" in the initial screenshot where I marked the 3 categories).