Understanding the exchange adjustment.

For Dynamics AX 2009, there is exchange rate on tables (i.e) : VendSettlement, VendTrans.

There is amountMST on bot tables, which represet the amout of current company currency.

Also there is amountCur,so

1) why is the field ExchangeAdjustment, and why it is not zero always (What may cause it to have a value)?

2) Is there any function that I can calculate the ExchangeAdjustment?

Thanks :)