Page 214 - Photoshop for Lightroom Users – Scott Kelby 2nd Edition

Step Seven:
The last two steps are just to help make our fix look more realistic. The first is to paint over the steps on the bottom left to reveal the original steps. That
way, the steps on the left side don’t wind up looking like an exact duplicate of the steps on the right side—so, you’re using the wall from the right, but
leaving the original steps on the left side visible. That’s what you’re seeing me do here—I’m painting in black over those steps to reveal the original steps
from the Background layer.