The default value set in this field is 2 decimals.
2. Set Currency Decimals
Please note that the decimals in the Salesforce Currency Setup can be configured to a maximum of 6 decimal places. If you have your Administration settings configured to show 7-9 decimal places, the calculations will take place based on 7-9 decimal places and the results will be shown rounded to a maximum of 6 decimal places in the UI.
To be able to show the decimals selected also in all fields you need to go to Setup > Manage Currencies and select for each Currencies you are working with "Edit".