RE: wsa:Action in responses

What does the clause "(and opaquely)" mean in the definition below? 

> 
> Actually, I see [action] associated with a message, not any 
> particular sending or receiving node: as the spec describes 
> it, it is "an identifier that uniquely (and opaquely) 
> identifies the semantics implied by this message."
> 

To whom is this property opaque? Am I right in assuming that it is opaque to the sending and receiving ***nodes*** but quite meaningful to the sending and consuming ***applications*** using this message for data interchange?

Thanks,
Nilo