The Sign Post is a buildable structure that allows players to leave messages of up to 140 characters. Sign Posts can be up/down voted, which will increase/decrease their lifespans. The builder of the Sign Post can edit the message after a short amount of time has passed. The distance at which sign post messages appear is configurable in the Options menu. The message also shows how long ago the post was built and the name of the builder.
It cannot be destroyed by gunfire and does not consume any supplies or suffer from decay. It doesn't block movement. Its blueprint doesn't block construction, you can build a pillbox on the exact same spot as an unfinished Sign Post blueprint.