how is action id converted to number?

Hi,

In definition of action window, I wonder how the id 'action_picking_tree4' is converted to number (in database, it is 324)? How does OpenERP know mapping between 'action_picking_tree4' and 324?

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<record id="action_picking_tree4" model="ir.actions.act_window">
            <field name="name">Incoming Shipments</field>
            <field name="res_model">stock.picking</field>
            <field name="type">ir.actions.act_window</field>
            <field name="view_type">form</field>
            <field name="view_mode">tree,form,calendar</field>
            <field name="domain">[('type','=','in')]</field>
            <field name="context">{'contact_display': 'partner_address',"search_default_availab
le":1}</field>
            <field name="search_view_id" ref="view_picking_in_search"/>
            <field name="help">The Incoming Shipments is the list of all orders you will receiv
e from your supplier. An incoming shipment contains a list of products to be received according
 to the original purchase order. You can validate the shipment totally or partially.</field>
        </record>

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Regards,