Identifies the reference page from the rate source.

For FX, the reference page to the spot rate to be used for the reference FX spot rate.
When RateSource(1446) = 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: https://www.fpml.org/coding-scheme/settlement-rate-option.