How to reference xxxdvd1.iso in sources.list

[Richard Owlett <rowlett@cloud85.net>]

I've seen a reference to using the ISO file of DVDnn in a sources.list .
I remember that it gets identified as being ISO9660 and labeled as 
trusted, But I can't find a detailed example. All I find are references 
to unpacking the iso to a directory which is then loop mounted. That's 
not what I'm looking for.
Pointers please.
TIA