[ietf-dkim] 2.1 Signers // Within an administrative domain?

[Douglas Otis]

,---
| 2.1  Signers
|...
| The key issue is that a message must be signed before it
| leaves the administrative domain of the signer.
'---

What is intended by this statement?  How does this relate to messages  
signed by an MUA, which is not mentioned as a possible signer?  Is  
this statement intended to preclude the use of MUA signed messages?

(Use of the word "key" appears to be a poor choice of words.)

-Doug