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GNU tar was based on an early draft of the
POSIX 1003.1 ustar standard. GNU extensions to
tar, such as the support for file names longer than 100
characters, use portions of the tar header record which were
specified in that POSIX draft as unused. Subsequent changes in
POSIX have allocated the same parts of the header record for
other purposes. As a result, GNU tar format is
incompatible with the current POSIX specification, and with
tar programs that follow it.
In the majority of cases, tar will be configured to create this format by default. This will change in the future releases, since we plan to make ‘POSIX’ format the default.
To force creation a GNU tar archive, use option --format=gnu.