Slitherine

Example:

Austrian 1809 Danube list. I bring in an attached heavy cavalry division from the reserve, consisting of 2 cuirassier units.

Can I also put into that division artillery from the main list? In other words, does an attached division have to consist only of troops from the attached division's list or can it also have unit(s) from the main list grouped with them into their division?

And the other way too - an attached division's unit in a main force division. For example a horse artillery from an attached cavalry division joining a main division.

My first thought is no. In the special rules it states attached units must follow their own list restrictions, which to me implies you are dealing with two separate entities.
Let's see what others have to say.

All the army lists I have looked at say something along the lines of "the army may take a division from the cavalry reserve list."

That leads me to believe that the troops from that list must be in a separate division of their own, rather than intermingled with BGs from the main list.

Marc

Although it's somewhat obvious that a division from another list should be together, some people might wish to push the boundaries of what is possible when it comes to producing lists for competitions. Loopholes like this need to be closed.

I don't think its a loophole. I think it is really clear. You get a division from another list, not a meld from other lists.

In general, the attached division must be a seperate division and cannot be mixed in a division with troops from the main list.
There are exceptions to this but only when as dierected by the customised army notes - for example the Saxon army of 1809 can have Wurtember or Bavarian cavary in an otherwise Saxon cavalry diivsion chosen from the main list.