Just from toying with it for a moment, I would guess that tmp values control PIPE particles' state. Like whether empty, whether connecting to 1px thick pipe, whether carrying something -and- connecting to 1px thick pipe, connecting to empty space, connecting from empty normal pipe to empty 1px thick pipe?
I haven't looked at the code though so, I don't really know. @_@ Usually people just dump whatever calculations they need into whatever variables a particle isn't using.
I know what its used for now, abd i figured it out a while ago.
Its mainly used to determine the element thats inside it. You can test this by putting the cursor over a filled pipe area. then putting the cursor over an empty pipe area then typing !set tmp pipe #, where the number = the tmp # of pipe, which will turn it into the element with the corresponding number.
Note: Some numbers go as high as 6000 and I've been able to put WIND in pipe by styping !set tmp pie 414. It does nothing but in theory, i guess thats whats left of the Formal WIND Element.
@OmegaSupreme(View Post) It's quite useful for making electronically-detonated DEST, BOMB or GBMB units. You just coat it with C4 and when the C4 is detonated by SPRK, the explosive is released, I've been using that for quite a while (at least a year).