Why # in URL is encoded to %23

Hi!
I am using ckeditor 3.3.1
When selecting an element from my custom File browser, the URL of the element is correctly sent back to the dialog box. I dont understand the reason why the character is '#' replaced by its ASCII value.

Can any one please explain why ckeditor replaces the # from URL?

//_source/plugins/filebrowser/plugin.js (Line311)
url = url.replace( /#/g, '%23' );