Check the Specification box if you want to make specifications on an account. A specification is an additional text, registered on a transaction row, and visible in the Print Journal/Print General Ledger etc.
This alternative can be used when coding foreign currencies. If you activate Specification for the bank account in the chart of accounts then the currency code for the payment being made will be placed in the spec-field in the coding and accounting. This will clarify which currency that the transaction concerns.