It's stacking restrictions for empty containers in lots that are not at the port, I believe.
> on-chassis (which I think means on wheels)
In container shipping, a chassis is basically a trailer that accepts a container and can be pulled by a semi-truck or one of the utility vehicles they've got at the port to move things around.
If a chassis comes back with an empty container on it, and the port isn't accepting empties, you've got to leave it somewhere before you can grab one of the many containers sitting at the dock that have goods waiting to be delivered. If the dock yard is full of containers, they can't unload the boats. If the can't unload the boats, they can't load outgoing cargo including outgoing empties.
Stacking empties higher, especially away from the dock may free up chassis that frees up dock space, etc, that gets things moving and then the empties can come back. But, that only works if the storage yards have the equipment to stack higher; which probably they don't all have. The thread mentions a limit on stacking empty containers, but the zoning limitation is for stacking any containers, it's just that outside of the yards at the port, you tend not to store full containers. Once you get those on a chassis, you want to get them delivered either to the final destination or a storage yard at another port or a train depot, etc. Empties are a bit different; you'd prefer to load filled containers most of the time, so it makes sense to stack some empties from time to time; also a trucking company may want to have some empties to take to an exporter, etc.
Honestly, I had thought that both chassis and empties were fungible, kind of like with rail wagons. You count what goes where and if things are uneven over time, make some transfers to bring it back, but otherwise no big deal. But the thread says otherwise.
