Pipes, or Warp Pipes, are entities to take a player to another sublevel. They are activated when the player crouches on their area of effect, which is the block the pipe occupies and the one to the left. As such, they should always be placed on the right-hand tile of a pipe. While they can be placed on any tile, it is considered extremely bad form to not place them on a pipe or Portal elevator, or otherwise indicate their presence.
Pipes simply take the player to another sublevel. They are by far the most common means of taking the player to another sublevel.
Pipe Exits Edit
Pipe Exits are where they player should spawn after taking a Pipe or falling from a Bonus Level, assuming it is from the correct sublevel. If it is in or to the right of a block, the player will come out of it like it's a pipe, otherwise they will simply spawn in the entity's location.
Warp Pipes Edit
Warp Pipes take the player to an entirely different level. However, if the level you set it to does not exist, it will say you have completed the mappack, and if it is set to World 4-1, it will instead send you to the Minus World. Other levels in World 4 will not do this.
The Warp Zone Text level flag is on, when you trigger the text to appear, the number of the world it takes you to will be shown above the pipe. The text will also allow a Warp Pipe to take you to World 4-1.