Access session variables outside OsCommerce

If I don't include
require('includes/application_top.php');
I can't see session variables like 'customer_id', etc.

I can't see any session variables which set by OsCommerce with
var_dump ($HTTP_SESSION_VARS);

After I include
require('includes/application_top.php');
I can't set session variable with
$_SESSION["my"]= "value";
unless I use OsCommerce functions.

If I set session variables before application_top.php, everything works fine.

Why are their session variables so special?
Thank you for your help in advance.
Frank