Responding to someone aksing about our unspoken rules for using email…

The uber convention is to respect privacy. There is the main recipient … TO: whom the subject applies. There are recipients in the Carbon Copy (or CC: ) and those who might wish to receive the information but do not want to be known or included in any subsequent: Reply All:  They are in the BCC: space.

Public Recipients
A.) TO:
In the case where you expect a response, are asking for a reply, or are at least hoping for one… use the TO: addressing space.

#1) Notice that a responder to the message has a choice, they can send back simply to the originator by “Replying” or to all recipients with the “Reply all”
But also: That anyone in the CC list can do the same as someone on the TO: line. See #1

B.) CC:
In the case were others have a specific, or prior expressed interest, or have an inferred personal need to know and want to be informed — but may have no participative aspect, use the CC: addressing space.

#2) Notice that a responder can choose to include these participants by choosing to “Reply all.” The responder can choose to leave them out … (see #1 above.)
But also: That anyone in the CC list can do the same as someone on the TO: line. See #1.

Non public Recipients:
C:) BCC
In the case where you wish to circulate this kind of content to undisclosed, or uninvolved and non-participative addresses; if your purpose is only to inform a list, you use the BCC address space.
BCC recipients can see and choose to email recipients in both of the above cases.

For a broader, and hastily updated treatment: Email use best practices 10-2013