Q: Attachments and IDs?

[David Masterson]

It's taking awhile to get through my hard head, but I think I'm
beginning to understand the directory structure of attachments.

  org-attach-id-dir/IDx/attachment-file

I don't understand the value of IDx. I would think that "attachment:"
could simply reference 'org-attach-id-dir' (drop 'id'?) unless
overridden by a DIR property.  I think I'm missing something that would
help my understanding of attachments.  Am I?

Is there a section of the Org manual that talks about IDs?

-- 
David Masterson