Open vs Closed Universes

Along with MajikkanBeingsUnite, I would like to work on getting the terms "closed universe" and "open universe" to be more widely known.  An open universe is one where interactions between characters from that species/group/setting can interact with other characters.  A closed universe is one where they can't.  This is separate from the status of a species, you can have open species with closed universes and closed species with open universes.  And anything else.

Some Closed universes I have:
pikumon 
CTnet (as an MMORPG AU to CardTamers, it makes a lot of sense)
CardTamers 
KimechARPG 

Some Open Universes:
HoodPups
WhirbetsWorld 

In addition, here are some stamps (they're also emoticons, search my username or "statusstamp") to use to distinguish in groups/projects:
:bademoticon: :bademoticon: :bademoticon: :bademoticon: 

EDIT: There are also stamps for semi-open and semi-closed universes. Semi-open universes are ones where they can interact with most other universes, you just need to ask the creator to approve it first.  Semi-closed universes are ones where they can interact with only certain universes, decided by the species creator.

Semi-Open Universes I have:
(None Yet)

Semi-Closed Universes:
PlateFighters (Can only interact with WhirbetsWorld characters)