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/.