What is the name of the software interface that is used to allow external hardware control surfaces devices to talk to Sonar?  

Found it... it's called ACT

Yep. Active Controller Technology.

MCU-Pro & a BCF2000

Hi Bill, I was actually looking for the actual software protocol that is used.   I see that Cakewalk has a ACT API, but does not look like there is much support around it.

Is there some other protocol that Sonar supports that is widely used in the DAW/Hardware Surface world?

ACT = Actually Controls - Temporarily.

Hey RB, you may be thinking of Mackie Emulation Mode? It's for control surfaces (thinking faders, knobs & buttons). It became the "standard" because Mackie was early in the whole control surface game, and their boxes became popular very quickly, and so many DAWs developed support for the protocol. The advantage in SONAR is that this mode is bi-directional, meaning that the control positions are fed back to the controller, which is the only way to do things like motorized faders and visual feedback with LED indicators.

Hey losguy,

This is along what I'm looking for.  So, is it safe to assume then that ACT is a Sonar specific mode, where Mackie Emulation Mode is a universal mode?  

ACT is indeed Cakewalk only. I have yet to see a non-Roland/Cakewalk device that touts direct support for it. Same with Novation and their Automap. (If someone does know of one, I'm curious to know.) Folks have used ACT on non-Cakewalk/Roland control surfaces, though as I understand the setup is known to be a bit "fussy".
 
"Universal" is a little strong a word for me to apply to Mackie Mode - it's not as universal as, say, MIDI or USB, where it would have been ratified by a committee of the MMA or IEEE, for example. But gosh-darn if almost every control surface out there doesn't support a Mackie Emulation mode, and of course, SONAR has a control surface plugin setting to support it too. Just the same, it's best to check the specs and manuals and tutorials of whatever device you intend to use, to make sure that it supports it.

not sure what you mean by support but there is a fair amount of setup information.
http://www.cakewalk.com/Support/kb/reader.aspx/2007013008