".jpo.tar" seems to indicate the file type or the compression format used. "jpo" might stand for a specific compression algorithm or a file format, although it's not widely recognized. The ".tar" at the end reinforces the idea that the file is a tarball, which is a collection of files and directories stored in a single file.
: The file format is a Compressed Archive (TAR), which contains the executable binary along with radio firmware and auxiliary files needed for the update. Core Functionality Ap3g3-k9w8-tar.153-3.jpo.tar
