https://powdertoy.co.uk/Wiki/W/Using_pipe_element.html
It contains all you ever wanted to know about PIPE.
It is tmp. Check the part of the wiki "Using Console Commands".
Setting tmp of pipe to different numbers makes it have different things inside it. !set tmp pipe 0 removes aall particles, 1 is dust, 2 is water and so on.
Meh. Too long for me. :/