Hi Anothai,
if Biao's suggested way to understand the logic is too technical, maybe the explanations in the SAP library are helpful:
http://help.sap.com/saphelp_erp60_sp/helpdata/en/42/c97df81d446baee10000000a1553f6/frameset.htm
->
Rate Valuation for Forward Exchange Transactions.
Note 184678 explains how the swap rates are read from customizing.
Hope this helps,
Marie