Identifies the reference page from the rate source.
When UnderlyingPaymentStreamNonDeliverableSettlRateSource(40661) = 3(ISDA Settlement Rate Option) this contains the value from the scheme that reflects the terms of the Annex A to the ISDA 1998 FX and Currency Option Definitions. See: http://www.fpml.org/coding-scheme/settlement-rate-option