C6D stands for Canadian 6 Degrees. Loosely defined, this is a large group of smaller related fandoms that have some connection to the Canadian television show due South.
Of interest to members of this community might be the fact that Callum Keith Rennie (Leoben Conoy) is a main part of the C6D fandom.
Please check out c6d_weekly's profile for more information.
If this is too OT, please let me know and I will delete. Thank you.