WordPress at a glance
filter is not described

comment_form_field_(name) filter-hook . WP 3.0.0

Filters a comment form field for display.

The dynamic portion of the filter hook, $name, refers to the name of the comment form field. Such as 'author', 'email', or 'url'.

Usage

add_filter( 'comment_form_field_(name)', 'filter_function_name_8248' );
function filter_function_name_8248( $field ){
	// filter...

	return $field;
}
$field(string)
The HTML-formatted output of the comment form field.

Where the hook is called

comment_form()
wp-includes/comment-template.php 2417
echo apply_filters( "comment_form_field_{$name}", $field ) . "\n";

Where the hook is used (in WP core)

Does not used.