Trying to find out more info on ATTACHED units. Although this is not an ENGINEER unit, this might help shed some light on a similar note:
How Did AAA Units Operate?
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Normally, an AAA automatic weapons (AW) battalion was attached to each division — a self-propelled (SP) AW battalion if attached to an armored division, and a mobile AW battalion if attached to an infantry division. A corps normally had one or more AAA groups attached. Each AAA group consisted of two or more automatic weapons battalions (usually mobile), although a gun battalion was occasionally attached. In the European Theater, gun battalions were more frequently found in AAA groups attached to the army or army group. Antiaircraft brigades were also formed and were normally attached to armies or to theater commands. In addition, the IX Air Defense Command (in effect an AAA division, originally organized as a part of the U.S. Ninth Tactical Air Force), with an average of 10 to 15 gun and automatic weapons battalions, formed a powerful AAA reserve for General Omar Bradley's 12th Army Group in Europe. Searchlight battalions, on the other hand, could be attached to either an Army or Air Force as part of an AAA brigade. The 225th, for most of 1944-45, was part of the 49th AAA Brigade and alternately attached to the U.S. 1st Army and the 9th Air Force, and wore shoulder patches for one or the other, or both. Rarely would an AAA man at the battalion level wear a patch of any brigade, division, or squadron to which his unit was attached.
This was taken from:
Proud Daughter of Walter (Monday) Poniedzialek
540th Engineer Combat Regiment, 2833rd Bn, H&S Co, 4th Platoon
There's "No Bridge Too Far"