They carry the same latching interface as the boxes, fixed to a van wall, workshop wall or bench, so units lock to the structure exactly as they lock to each other. The stack becomes part of the vehicle or building rather than loose cargo that slides and topples.

Load security is a legal obligation on the driver in most jurisdictions, and an unrestrained stack of loaded tool boxes is a significant mass in a collision or hard stop. Mounting is the practical way to meet it for modular systems, and it also prevents the shifting that wears latches and cracks cases.

Structural members wherever possible, not van skin. Panels are thin, and a plate fixed into sheet alone will pull out under load precisely when it matters most. Spreader plates, backing washers or plywood backing are the usual approaches where a structural member is not in the right place.

No - they assume a sound substrate and correct fixings. A rating is a property of the plate, not of your van wall. The weakest element is almost always the fixing into the vehicle, so the installation should be assessed on that rather than on the plate's published figure.

That is the main practical benefit of wall rails. Boxes hang at working height in the shop, are lifted straight out and latched into the van for a job, and returned afterwards - so tools are never unpacked and repacked between the two, which is where things get lost.

Around the units actually owned and the way the vehicle is loaded and unloaded, rather than a catalogue arrangement. Consider what is reached from the rear doors against the side door, leave room for the units to be latched and released without fouling, and keep heavy items low for vehicle stability.

Generally not, since they carry the manufacturer's own interface. Some third-party rails and plates support more than one system, which is worth investigating for a mixed fleet, but the safe assumption is that plates must match the box system already owned.