what is the use of form[#special] in a drupal form?
You can embed anything in your form. Just add # at the beginning of the array index to indicate it's not a child element, and Form API will accept that even if it does not define or use it. Form API reference does not mention #special, so it is not an standard parameter. Instead, it's a parameter created by module's author for his own need.

For example, I usually add #machine_name to anything that can trigger AJAX, for easier handling of $form_state['triggering_element']. There are many uses.