FMP 13 uses the same file format as FMP 12 (i.e. .fmp12).
However, do not reopen a file created in version 13 with the old FMP 12. There is place in the File Options to prevent this by setting the minimum version to 13.
good to know... thanks!
Actually, you may want to be able to open the file from FileMaker 12. It's not like your file self destructs or something if you do, it's just that a number of new features in 13 will not work or display correctly in 12. So it will depend on the design choices that you make whether or not you need to prevent the file from being opened in FileMaker 12.