PaxHeaders in tarball
POSIX.1 2008 defines an extension to the ustar format called the pax format. It's new features are introduced using special files with typeflag set to x for an extended pax header (affects the next file) or g for a global pax header (affects all further files). These headers allow archiving programs to store additional attributes in tar files, like access time or extended length (to store file too large for conventional tar).
Space in this answer is too restricted to replicate the full specification, read POSIX for more details.