On most systems where FIT2D is installed and updated a previous version is saved in the same directory as the current FIT2D program. The old version is called fit2d_old. If ever you find that a bug has appeared in a new version which was not present in the old version, you can use the command fit2d_old instead of fit2d to obtain an older version. This will take all the command line options exactly as for fit2d. (It cannot be guaranteed that sites outside the ESRF are set-up like this, but this is the recommended set-up.)
At the ESRF new features may be added to a development version of FIT2D during an experimental run owing to user requests. Since it is preferable not to change the standard version during a run, normally any changes will be made to a version called fit2d_new. Only if a major bug is discovered (one which is likely to affect many users) will the standard version (fit2d) be changed during a run.
Between experimental runs, the standard version may replace the old version, and a new version may replace the standard version.
(An external site may also choose to set-up a fit2d_new version.)