In software distribution, a "repack" is not an official Microsoft ISO. It is a third-party modified version of the operating system. Repackers (individuals or groups) take an original ISO and alter it.

However, downloading and installing a repackaged ISO also carries potential risks:

Windows 10 1607 is a nostalgic and efficient build, but downloading "repacks" from untrusted sources is risky. Stick to untouched ISOs and customize them manually for the best balance of speed and security.