How specifically is FCCS_Mvmts_FX_Movement calculated

Content

Hello,

I cannot find enough detail in the admin guides regarding the member: 'FCCS_Mvmts_FX_Movement'. Best i can see is: 

Foreign Exchange Variances (FX Opening, FX Movements) are then calculated to bring the aggregated Closing Balance to the equivalent of the untranslated Closing Balance translated at Ending Rate.

or

FX Movements stores the difference between the translation of total movements at the Ending Rate for the current period and the translated total movements using the Average rate or historical rate or amount overrides for the current period.

however:

'FCCS_Mvmts_Subtotal' @ CurrMonth End Rate -  'FCCS_Mvmts_Subtotal' @ CurMonth Average Rate