ORIGINAL: geofflambert
Well, it's correct some of the time or most of the time. And the ones that it is the case for cannot under any circumstances have the same aircraft. For instance, the DET can only have recon aircraft but the parent can only be bombers.
Yes, that is one example of a no-go situation.
Open the editor and look at the IJN air group line #007 (scenario 1 stock).
Chitose Ku S-1 Det in the "parent" line has a box marked "6", and next to it displays the parent as Chitose Ku S-1, which is on line #006.
These two units start flying Claudes on Roi-Namur and Maloelap. If you fly them all onto one island, the detachment can be reattached. (I just tested that to make sure).
Here is another interesting point - the parent formation has a withdraw date, 450505, and a return marked 9999, i.e. it's a "P" withdraw. The detachment has no withdraw date set. However, when you open up the detachment group screen at Maloelap, it also is marked "(Remove by 5 May,1945, P)", so it picks up the parent date. Very neat coding.
Now please shift attention to line #039 - Kanoya Ku K-1 Det. This has no parent formation set. The Kanoya Ku K-1 group is in line #038. These units have different HQs but are both flying the G4M-1. If you fly the detachment down to Saigon where the not-parent is, change its HQ to 22nd Air Flotilla, you confirm what the absence of a parent formation in the group settings already tells you - they don't re-attach. Further examination of the unit settings in the editor does tell you that the Kanoya Ku K-1 Det withdraws on 420310, and a few weeks later on 420401 the not-parent gets an upsize from 27 to 36 airframes (still less than the combined number in the two starting groups). So the air team were doing something different with that group/detachment to get to that end result and take it out of the players' hands. More about group sizes later.
While you are in Saigon, check out the Genzan KuK-1. The "Unit OOB" button is lit up and tells you there is a detachment, also at Saigon. It's re-attach button is available because the child has a parent nominated on the group screen in the editor, everyone is flying the same plane, and has the same HQ. Change the HQ to 21st AF, and the reattach button disappears. Change it back to 22nd AF like the parent and it is available again.
Bottom line is, you could have a uniquely named air group called the "Taranaki Girl Scouts Gliding Club" or something, but if it has a parent unit set in the editor, if it has a common aircraft type set, and if it has the same HQ, it will reattach to the parent. After it reattaches, whether the parent can split again is dependent on a different unit setting - the max splits. In the above examples, the Chitose Ku S-1 has max splits set at ZERO, so once reassembled it stays that way.
Last point - the maximum airframes is whatever the parent is set to in the editor, minus the size of the detachments. The Chitose Ku S-1 has a maximum of 27 aircraft (changes to 36 in 1942). In stock, the detachment at Maleolap has a size of 9 aircraft, and that is deducted from the parent maxium size. Likewise, with the various late game USMC CVS groups that have a fighter squadron, possibly a 9 airframe night fighter detachment, and a 2-plane recon detachment, the maximum operational machines the parent can use is its maximum size, less the combined size of the detachments, and that is noted on the airgroup screen up near the top left hand corner.
Hopefully that helps clear away the fog. It took me a while to get across all the nuances of this when I was building my modified scenario, but there is a mathematical logic to it which at some point "clicks" and it becomes intuitive.
Edit: Punctuation.
