Our PU3000/4000 line features a shiny side and a dull side. In most cases, it is preferred to have the PU (shiny) side of the fabric facing in.
For a tent floor or ground cloth, that means you want the PU side facing up, away from the ground.
When used as a tarp, you want the PU side of the fabric facing inside.
When seam sealing PU3000/PU4000 materials, you want to use Seam Grip on the PU side.