A door sill is the steel member that runs across the bottom of a container's door opening, at floor level where the two doors close down onto it. It is the lowest point of the door frame, it is where the bottom cam rod keepers and the door gaskets seal, and because it sits at ground level it takes more moisture and wear than any other part of the frame.
Stand at the back of a used container and look down at the sill, and it is easy to treat it like every other patch of rust on the shell: cosmetic, brush it off, move on. That is usually a fair call for the walls and the roof. It is a worse assumption at the sill, because the sill sits where standing water and gravel actually collect, and rust there has a shorter path to something that matters.
The deciding factor is not whether there is rust. There almost always is some. It is whether that rust is surface, the kind that flakes off with a wire brush and shows solid steel underneath, or scale, the kind that has eaten through in layers and leaves pitting or a soft spot you can push a screwdriver into.
Press a screwdriver or a key against the sill at a few points along its length, especially where it meets the corner castings. Solid steel stops the tool cold. Steel that has gone soft or crumbles gives. If the container is up on blocks, look at the crossmembers underneath too, since a rusted sill and a rotten crossmember behind it usually show up together, and the crossmember is the one actually holding the floor up.
A sill with surface rust and a straight seal against the doors is a normal used container, not a red flag. A sill with scale rust, a wobble where the door will not seat, or a soft spot underfoot is the kind of thing that changes what a box is worth, or whether it is worth buying at all.
Every other part of the door frame sheds water. The sill collects it. Rain runs down the doors and off the roof edge, and gravity sends whatever gets past the gasket straight to the bottom rail, where it sits against steel instead of running off. Add gravel kicked up by loading equipment and salt off a winter road, and the sill simply sees more of what ages steel than the header ever does.
That is also why leveling matters here specifically. A container set on a pad that drains, even a simple gravel bed with a slight pitch away from the doors, keeps water moving instead of pooling at the one part of the frame that is already working hardest.
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