There is only one file type that uses the .USTAR extension:
Uniform Standard Tape Archive Format File
Archive file created using USTAR formatting, which defines a small extension to the .TAR format; stores a collection of files in an uncompressed format; extends the TAR specification with a longer header definition, support for longer archived filenames, and support for more information about each file in the archive.
USTAR archives are supported by many modern applications that also open TAR files.
Updated: February 3, 2011