For example, say I, as Remus, update something about how being a werewolf sucks. It's something Remus would want to get out, and also something the other Marauders might want to talk to him about. However, if, say, Lucius read it and went "Oh snap, that Lupin prat's a werewolf," it would be a pretty bad scene. To prevent Lucius' character or any of the other characters from knowing that, I would label it as "Marauders Only" or something akin to that. That way, all the roleplayers know that the material under said label could only be read and known about by the marauders.
The same goes for any other character. If Lucius goes "God I'm so cool. I mean look, I'm a death eater!" and he only wants Slytherins to know, he could say "Slytherins only."
Of course, that doesn't mean the roleplayers can't read it. Feel free to. It's just, you know, your character can't. :D
Hope that clears a few things up.