There is only one file type that uses the .TAX13 extension:
TurboTax 2013 Tax Return File
Alternate file extension used by TurboTax for 2013 tax returns; contains federal and state tax forms as well as data entered by the user; can be electronically submitted to the IRS and state tax authorities when completed.
NOTE: TurboTax 2013 saves tax returns with a .TAX2013 file extension by default. However, the program recognizes files saved with the shortened ".tax13" extension as well.
Updated: November 24, 2016