Trellises are like thin iron bars that can occupy 6 sides of the same block space.
Placement
When trying to place, the side that is going to be placed is the one nearest to the vision line.
If the side is already occupied, it will attempt to place the next closest one.
When sneaking and attempting to place a trellis on the same block,
it won't fill more sides to that trellis block and instead attempt to place the trellis on a new block space instead.