WordPress at a glance
filter is not described

default_content filter-hook . WP 1.5.0

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

Usage

add_filter( 'default_content', 'filter_function_name_3465', 10, 2 );
function filter_function_name_3465( $post_content, $post ){
	// filter...

	return $post_content;
}
$post_content(string)
Default post content.
$post(WP_Post)
Post object.

Where the hook is called

get_default_post_to_edit()
default_content
wp-admin/includes/post.php 662
$post->post_content = (string) apply_filters( 'default_content', $post_content, $post );

Where the hook is used (in WP core)

Does not used.