restrictions on use of xs:all?

I'm trying to get a better handle on exactly what is and what isn't allowed
with xs:all. I used to think that elements that used it in their content
model couldn't be nested inside of other elements, but I see now that this
is wrong. 
 
Correct me if I'm wrong: because its use must be at the top level of a
content model, we can define a content model like (a & b & c & d), but not
(a, (b & c & d)), right? 
 
thanks,
 
Bob DuCharme