What is the hash added to the product_id in the shopping cart?

[torvista]

Here is the contents of the $products array, extracted from the shopping_cart header for a simple product (no attributes) in the cart.
I understood that for a product with attributes, a hash is used with the pid to identify the attributes.

But for a simple product, what is this hash shown here, and from where does it get added? Somewhere in main_cart-actions?

$products (array):
Array
(
    [0] => Array
        (
            [id] => 25:8e296a067a37563370ded05f5a3bf3ec
            [category] => 8
            [name] => Microsoft Internet Keyboard PS/2
            [model] => MSINTKB
            [image] => microsoft/intkeyboardps2.gif
            [price] => 69.9900
            [quantity] => 2
            [weight] => 8
            [final_price] => 69.99
            [onetime_charges] => 0
            [tax_class_id] => 1
            [attributes] => 
            [attributes_values] => 
            [products_priced_by_attribute] => 0
            [product_is_free] => 0
            [products_discount_type] => 0
            [products_discount_type_from] => 0
            [products_virtual] => 0
            [product_is_always_free_shipping] => 0
            [products_quantity_order_min] => 1
            [products_quantity_order_units] => 1
            [products_quantity_order_max] => 0
            [products_quantity_mixed] => 0
            [products_mixed_discount_quantity] => 1
        )
)