can someone explain this part of ^TO?

^TO expands to:

(^((Original-)?(Resent-)?(To|Cc|Bcc)|
(X-Envelope|Apparently(-Resent)?)-To):(.*[^a-zA-Z])?)

What sense does it make to have the 'Bcc' in there?

The Bcc never shows up in the header, right?

TjL