There are basically two/Three currrencies for every company.
1. transaction currency(Amount Cur)
2. company currency(Amount MST)
3. secondary currency(Optional)
As You know that, transaction currency is the currency through which the posting is done or the currency specified while posting a record. To check the values in that company based on transaction currency we use AmountCur field in almost all transaction tables.
As You know that, company currency is the currency specific to that company. To check the values in that company based on company currency we use AmountMst field in almost all transaction tables.
As You know that, secondary currency is the companies secondary currency. It is used to maintain the values specific to that company in a currency other than the company currency. It is also called as secondary company currency.
In AX 2009:
Exchange rates are Provided in GL > SET UP > Exchange rates
In AX 2012:
Exchange rates are Provided in GL > SET UP > Currency > currency Exchange rates
Use Currency::amountCur2MST() to convert transaction currency to company currency.
Currency::curamount() to convert company currency to transaction currency.