A shared footprint and latching interface, so any unit locks to any other and the stack moves as one object. The box itself is ordinary; the interface is the product. That is why systems are bought into rather than bought individually, and why the choice matters more than a single toolbox purchase would.

No. There is no common standard, and each manufacturer's footprint and latch pattern is its own. Adapter plates exist for some combinations but are a workaround. This means the system choice is effectively a long-term commitment, with the switching cost growing as more units are added.

As few as the work allows. A stack of matched footprints and similar depths loads and packs predictably, whereas an assortment of odd sizes wastes space and makes the stack unstable. Most users settle on two or three unit types plus one machine-specific case rather than one of everything.

Strength, and whether they can be worked with gloved or cold hands. Latches are what fail on modular systems, because they take the whole load when a stack is lifted and are operated dozens of times a day. A stiff latch that needs bare fingers is a daily irritation that eventually stops the system being used properly.

Ratings vary and should be checked rather than assumed. Many carry a sealing gasket and an ingress rating adequate for rain and site dust, which matters if boxes travel in an open truck or are left outside. Full immersion resistance is rarer, and a rating claimed for the box does not always hold once it is stacked.

In practice it always is, so lid rigidity is worth checking even though it is rarely specified. Some systems provide a purpose-made work top or a lid designed to take load; on others a heavy workpiece will flex or crack a lid that was only intended to close.

Manufacturers publish a figure, and it matters most for the bottom unit in a tall stack, which carries everything above it plus whatever the stack meets when a van brakes. Check the rating against a realistically loaded stack rather than the nominal weight of empty boxes.