Need to know how does sales order form fires defaults rules

Hi All,

Need to know how does  sales order form fires defaults rules. Is it at each field and how does it call the API.

when sales order form open is it calling the ont_header_def_hdlr.g_record to defaults all the value. when customer number entered is it firing again to get the defaulting values. Cause currency code is different when enter customer number but it get changed when i enter the order type. Please let me know how it works.

Thanks in advance.