WordPress at a glance
filter is not described

private_title_format filter-hook . WP 2.8.0

Filters the text prepended to the post title of private posts.

The filter is only applied on the front end.

Usage

add_filter( 'private_title_format', 'filter_function_name_3196', 10, 2 );
function filter_function_name_3196( $prepend, $post ){
	// filter...

	return $prepend;
}
$prepend(string)
Text displayed before the post title.
Default: 'Private: %s'
$post(WP_Post)
Current post object.

Where the hook is called

get_the_title()
wp-includes/post-template.php 145
$private_title_format = apply_filters( 'private_title_format', __( 'Private: %s' ), $post );

Where the hook is used (in WP core)

Does not used.