There are a few important differences between To, Cc and Bcc
With Cc, you inform others besides the main recipient.
Then the main recipient knows that that message was copied to other persons too, and the other person(s) know that they were purily 'informed', and they are not specifically required to respond to that message.
So in our situation an example would be: I ask you a question, Cc it to another member of who I know is also concerned with that subject.
- You then know the other member is also informed. And you are able to Cc him too when answering so everybody is up to par about the subject.
- The person Cc'ed then is informed about the subject being under discussion, but he is not specifically required to respond.
With Bcc, it is about secrecy.
The recipient will never know the message was shared with somebody else.
I can only think of using that when 'warning' or 'reprimanding' a forum member in the capacity of moderator, and you want to inform other moderators about it.
This is common practice in e-mailing. With fora, I am not a big pm'er myself, so it could well be that this might work slightly different there.
But I would assume it should work the same.
Also note the correct capitalization of Cc and Bcc I used in this posting ;-)