A 53′ dry van has about 636 inches of usable deck at roughly 100 inches wide. A GMA pallet is 48 × 40 inches. Loaded the way forklifts usually want them — 48 inches along the trailer, two across (96 inches of the 100) — you get 13 rows of 2: 26 pallets. Turn them so the 40-inch side runs along the trailer and two pallets take 48 inches of width each, and 15 rows of 2 fit: 30 pallets, with 36 inches of deck left over. That is the whole of the "pinwheel" legend; it is really just which way the pallet is turned, and whether the shipper, the forklift and the load's stability allow it.
The planner searches both orientations row by row, so the number it prints is the best straight-row arrangement for your exact deck and pallet — and it will not beat physics: weight, axle limits and whether the load can be stacked all cap the count. Double-stacking only helps when the freight is stackable, the interior height allows it and the bottom pallet can carry the top one.
Balance. The axle split is a beam calculation: the trailer sits on the kingpin and its own bogie, so the kingpin carries trailer weight × (kingpin-to-bogie distance − distance from the kingpin to the load's centre) ÷ kingpin-to-bogie distance. Move the freight's centre of gravity forward and the drives get heavier, move it back and the bogie gets heavier. The tool shows the estimate and the distance to move it, but the scale is the only authority.
Nothing here replaces securement rules: the aggregate working load limit of your tiedowns, load placement and the shipper's paperwork are all still on you.