See tar(1) for description of the -f function modifier.
If the TAPE environment variable is not present and the value of one of the arguments is a number and -f is not specified, the number matching the archiveN string is looked up in the /etc/default/tar file. The value of the archiveN string is used as the output device with the blocking and size specifications from the file.
% tar -c 2 /tmp/*
Writes the output to the device specified as archive2 in the /etc/default/tar file.