"4. Identify your target, and what is behind it." is short for wherever the bullet or shot will end up, be it deflected if you're shooting at a surface at a low

"4. Identify your target, and what is behind it." is short for wherever the bullet or shot will end up, be it deflected if you're shooting at a surface at a low angle, or in an arc coming down. For shotguns, it's fundamental to be aware of where the shot comes down.