[VOIPSEC] Differences between Middleboxes (in MIDCOM context) and SBCs?

[Michael Billerbeck]

Hi,
 
I'm confused about the terms Session Border Controller and Middleboxes.
Both are described in an abstract way as "intermediary devices".
But what makes them distinct? Is it that SBCs are designed especially for
the SIP-Protocol and Middleboxes are VoIP protocol independent by
abstracting the agent functionality?
When reading the informational draft
http://www.ietf.org/internet-drafts/draft-ietf-sipping-sbc-funcs-03.txt
on the functions of SBCs I'm getting quite confused.
When reading "Often, these proprietary solutions are implemented in network
intermediaries known in the marketplace as Session Border Controllers (SBCs)
because they typically are deployed at the border between two networks. The
reason for this is that network policies are typically enforced at the edge
of the network." 
Where are the significant differences that justifies these distinct terms?
Maybe this has to be differentiated in this draft.

Michael