The Format Code field in a field definition allows you to specify a custom formatting code to use when the number of date is displayed.

For more information on how these codes can be used, refer to the Microsoft documentation found here (for number formatting):

http://msdn.microsoft.com/en-us/library/0c899ak8.aspx

And here (for date formatting):

http://msdn.microsoft.com/en-us/library/8kb3ddd4.aspx