Q:

what is the use of form[#special] in a drupal form?

Now i'm working with forms and trying to add template in to it, I have form['#special'], but i don't know how to set it and what's the use of that and its usage.

A:

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.

