what is the difference if i select host machine or not when i flash my tx2 by jetpack4.2 in step 1
Host machine can also have CUDA, plus dev tools like the Nsite edition of Eclipse and cross compiler setup. If you choose sample programs on the Jetson, then cross compiler is used from the host, so choosing samples implies choosing host side tools.
A perfectly normal install would be first to install only the base system to the Jetson. And then on a later install, if you wish, install components to host. Later, pick CUDA and other packages for just the Jetson. You just have to remember the base install is first, and then you can add whatever parts you want without flashing again if you uncheck flash.