Chris Messina
CC is used to notify one or more Twitter users that a message might be of interest to them.
CC'ing ("carbon copying" in historic terms) is a useful way to point a tweet at someone so that they see it. It's a more intentional way of performing a mention without directly addressing the target. It is designed to mimic email's CC: field.
To use a CC "field", one simply needs to insert "cc @username" into a tweet:
Better yet, remove the parentheses and use the slasher instead, to save yet another character!
More Slashtags