Private/open for all

I'm slightly confused.... When people don't write (private) or (insertcharecternameshere) after their thread titles, is it then, an invitation for all, or? Some of the threads seem rather private, but are they? It's hard to keep track of when people are starting a new storyline and wants other to jump in, and when they're following up on 10 other thread.
Does it have anything to do with chance? For instance the thread The enemy of the enemy isn't marked as private, but it's fairly obvious that it would make no sense for Michel to join in, merely because the odds of accidently finding them are very very small. On the other hand there might have been another vamp following Sorin, for whom it would make sense to join, so it sorta makes sense that it isn't marked as "private".

Now, let's take the Lost in Nachton thread. Was Kyle and Mai joining that thread purely accidental? Might have been some pm'ing going on, perhaps Aishe and Mai know each other on some other board. I have no way of knowing, and thus I keep my chars away.

Perhaps if people used the (Open) thingy after their thread titles, to indicate that "Hey! It's ok! You can join here! I actually -want- you to!", it would all be easier? Or something...perhaps I'm the only one seeing a problem here, which I then suggest you tell me.

(The threads mentioned in this post weren't taken to pick on anyone in particular, but rather taken to illustrate a point.)