The 408th would be better attached directly to a corps or Army HQ. An army can have 12 asset units attached directly to it, and a corps can have 24. While attached to either of those HQ, the units do not count against formation attachment limits, but can trace GenS directly to any formation HQ subordinate to the army or corps the unit is attached to.
The 408th is actually a Brigade that would have been attached as an army or corps assets to support subordinate formations.
Section 15.0 speaks army and corps asset units.
Great, that was very helpful...
Still learning the in's and out's.