InstallShield PKG
From Just Solve the File Format Problem
InstallShield PKG is a format usually encountered as a file named setup.pkg. Such a file is included in some installers made by the InstallShield installation software for Windows.
Discussion
Authoritative information about PKG's purpose has not been found. It's sometimes described as "InstallShield compiled setup package". It is a binary file, relatively small, and appears to primarily contain filenames.
Identification
PKG files are observed to start with bytes 4a a3
, and have 02 00 00 00
at offset 6.