Attachments in their native format are actually encoded inside the message. Email programs will detect them and replace them with an icon or remove them entirely and show them in an attachment list for easier access.
Some email programs will remove the attachment from the message before storing it locally, and place it in a different sort of location. This may be a folder, a separate database, or a record in a different place in the same database.
Entourage for example, at least one version of it, stores attachments inside the main database. If that DB gets over a gig in size, it self-destructs. Bad news if you are holding onto a lot of large attachments. (have seen it repeatedly)
I don't see any way to describe this as a "holding station". If you send an email to a group with an attachment, the attachment is encoded into the email, and that email is sent individually to each recipient.
Interestingly enough, if you attach a folder of things in Apple's Mail, you see the folder in the email. You can double click it to open the folder in Finder. That leads me to believe that at least some attachments in Apple Mail are removed from the message for storage in a non native format.
