Excon letter

Am applying for a home loan with Nedbank. Now they have asked for a an Excon letter which they say i should get from my bank.After going to read about it its actually Exchange Control (Excon) or also now known as SA Reserve Bank (SARB) Financial Surveillance (FinSurv) heavily regulates the flow of funds.Reading further i discovered that its a regulation of flow of funds into South Africa.Am a permanent Resident in South Africa and all my income is made here which is seen by proof of income ie letter of employment.Can someone explain to me what this letter is for.Would appreciate if someone call me on ********** 694