How do I use Corps Attachments?

[Embis]

I have never paid any attention to Corps attachments, mainly because i don't really understand how I might make use of them.   The manual doesn't seem to shed much light describing how to do it but not why to do it.

I decided to explore the concept in game but didn't get very far.

As an example; I opened up the "getting started" file of Normandy '44 and saw that Corps attachments were possible in the opening turn. I invoked the Corps attachments menu item and was presented with the Corps attachments dialogue box. A bewildering array of organisations!!

Its easy enough to see how I can attach units to Corps, but:

how do I decide who to attach to whom? 


I presume that by attaching a unit to a corp that unit will derive benefit from the command and supply structure of the corp it is attached to, 

So what happens to them if I don't make attachments? do they run out of beans and bullets and wander about the battle field accountable to no-one?   

Any pointers as always gratefully received.