Dirt Wall

The dirt backdrop already lining a cave does nothing for you. Enemies spawn happily in front of it and a room built against it will not register as a valid house. Walls you put up yourself do both jobs, which is the whole reason you have to craft these out of dirt rather than knocking down what is already there. The two versions look almost identical but not quite, so a glance tells you which stretch of tunnel you have already sealed.

Shimmer converts a stack into the unsafe variant, which exists purely for builders who want an untouched cave look without accidentally creating housing. That is the opposite of what you want when walling off an arena or a spawn suppressed farm, so keep the two stacks clearly apart in your inventory.