On Nikon Cafe it's loosely structured, nothing formal, just a fun activity for whoever wants to jump in whenever they see an image and think, "oh, I've got something that would be perfect for this!" and add their own. No one is responsible for "maintaining" or supervising the thread, it's not like a competition or anything, and it's not bound by specific rules the way the POTD thread is. Now that I'm thinking about this, though, yes, I suppose at some point a long time ago an Admin or Mod set things up so that the thread would be permanently "pinned" so that it would be readily visible in the subforum in which it resides.