This is the extension for Final Draft 7 / 6 template files to differentiate them from the standard .fdr document files. FDT files are read-only and are designed to be used over and over again as stationery. If you save your file as a template [FD 7.1x] or as stationery (FD7 / 6 / 5), it will save with the .fdt extension.
For information on how to save a script as a template, click HERE.