What is the format of a .xip file
According to the xip manual page:


  A XIP file is an analog to zip(1), but allows for a digital signature to be applied and verified on the receiving system, before the archive is expanded. When a XIP file is opened (by double-clicking), Archive Utility will automatically expand it (but only if the digital signature is intact).


Essentially, an .xip file is just a .zip with a signature to verify that the file has not changed since its creator saved it. This protects from both damage from a disk error and from a third-party tampering with the file.