WordPress at a glance
filter is not described

comment_form_fields filter-hook . WP 4.4.0

Filters the comment form fields, including the textarea.

Usage

add_filter( 'comment_form_fields', 'filter_function_name_3768' );
function filter_function_name_3768( $comment_fields ){
	// filter...

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

Where the hook is called

comment_form()
wp-includes/comment-template.php 2372
$comment_fields = apply_filters( 'comment_form_fields', $comment_fields );

Where the hook is used (in WP core)

Does not used.