[ietf-dkim] Base-02 signature process clarification requested

[Douglas Otis]

,---
|3.5 The DKIM-Signature header field
|...
| The "DKIM-Signature:" header field is always included in
| the signature calculation, after the body of the message;
| however, when calculating or verifying the signature, the
| value of the b= tag (signature value) MUST be treated as
| though it were the null string. Unknown tags MUST be signed
| and verified but MUST be otherwise ignored by verifiers.
'___

What is not clear is when a DKIM-Signature is included within the h=  
parameter, for this instance of the header field is the b= tag also  
treated as a null string?

-Doug