What are the special cases in which double posting is allowed?

[Typhlosionvsworld]

have no idea what they are. Some help?

[Chibi_Muffin]

I think that one of them is when it's a fanfic/fanart thread, and you started it, and you're posting after one of your own posts to update with more chapters/art. There may be more, but this is the first example that comes to mind.