WordPress at a glance
filter is not described

default_title filter-hook . WP 1.5.0

Filters the default post title initially used in the "Write Post" form.

Usage

add_filter( 'default_title', 'filter_function_name_1328', 10, 2 );
function filter_function_name_1328( $post_title, $post ){
	// filter...

	return $post_title;
}
$post_title(string)
Default post title.
$post(WP_Post)
Post object.

Where the hook is called

get_default_post_to_edit()
wp-admin/includes/post.php 652
$post->post_title = (string) apply_filters( 'default_title', $post_title, $post );

Where the hook is used (in WP core)

Does not used.