SlackBuildsOrg/system/xar

xar (eXtensible ARchiver)

The XAR project aims to provide an easily extensible archive
format. Important design decisions include an easily extensible XML
table of contents (TOC) for random access to archived files, storing
the TOC at the beginning of the archive to allow for efficient
handling of streamed archives, the ability to handle files of
arbitrarily large sizes, the ability to choose independent encodings
for individual files in the archive, the ability to store checksums
for individual files in both compressed and uncompressed form, and the
ability to query the table of content's rich meta-data.

xar is used primarily for software install packages (.pkg files) for
macOS X.

To actually extract a .pkg file, do something like:

$ xar -x -f filename.pkg
$ zcat base.pkg/Payload | cpio -iv

You'll end up with Applications/ Library/ Resources/, etc.