Comment by rgovostes

Probably too late now, but xar already stands for "eXtensible ARchiver" and is a file format used on macOS in some package installers. It's notable for having an embedded XML "table of contents" that describes metadata of the archived files, so new fields can easily be added while maintaining backwards compatibility. (Compared to, say, the zip file format which does not even specify how to store Unix file modes.) https://en.wikipedia.org/wiki/Xar_(archiver)