In this context it means preserving the default behavior of running the command specified in the image as PID 1, and not injecting any extra processes in the na

In this context it means preserving the default behavior of running the command specified in the image as PID 1, and not injecting any extra processes in the namespace. The difference in the runtime environment is subtle, but some applications rely on it. If we changed this default those applications would behave differently after upgrading to Docker 1.13 or later.