WordPress at a glance
filter is not described

comment_form_default_fields filter-hook . WP 3.0.0

Filters the default comment form fields.

Usage

add_filter( 'comment_form_default_fields', 'filter_function_name_5621' );
function filter_function_name_5621( $fields ){
	// filter...

	return $fields;
}
$fields(array)
The default comment fields.

Where the hook is called

comment_form()
wp-includes/comment-template.php 2236
$fields = apply_filters( 'comment_form_default_fields', $fields );

Where the hook is used (in WP core)

Does not used.