Sign

A placed one holds text and shows it when you look at it, so it is the tool for labeling storage rows, marking which shaft goes where, and leaving directions in a build other people will walk through. It mounts on top of blocks, against their sides, hanging from their undersides, or flat on a background wall, which means you can label a room without anything sticking out into the walkway.

Each one takes a fixed number of short lines, so anything longer than a couple of sentences needs a row of them side by side. There is also a per-world cap on how many can carry text at once. It is generous enough that no ordinary build will come close, but a long-running adventure map can, and beyond the cap new ones will not open for editing at all until you remove some of the older ones.