When you stack pipe, you place a layer of pipe, then place boards called "stickers" crosswise to provide an even surface for the next layer of pipe. Frequently on flats you will see the ends of the stickers wired or srapped to each other to help keep the pipe from shifting. If you don't sticker the load, the upper layers try to slip between the lower layers and force them to move sideways.
Stickers are also used when stacking green sawn lumber to promote air circulation between the boards. Sometimes on lumber you see a crosswise stripe of discoloration lleft by these, which is called "sticker burn"
Philip