Why does the separator "::" work with headlines?

Example:

* Headline with separator :: This would be the description of a list, 
but what is it here?

> 2.6 Plain Lists
>
> /Description /list items are unordered list items, and contain the 
> separator ‘::’ to distinguish the description term from the description.
>
> - Sean Astin :: He plays Sam, Frodo's friend.  I still remember him
>      very well from his role as Mikey Walsh in /The Goonies/.
>