ISPM 15 is the international standard for wood packaging, including pallets, crates and dunnage, that crosses borders. It exists to stop pests travelling in raw wood.
Coming into the US
USDA APHIS says regulated wood packaging material entering the United States must be treated and certified under ISPM 15. Its page lists the problems inspectors find: packaging that is unmarked, and packaging that is inappropriately marked, where the stamp is illegible or doesn't comply with the standard.
What isn't covered
APHIS lists materials that are not subject to its wood packaging rules, including plywood, pressboard, oriented strand board, hardboard and plastic pallets. For a shipper that is often the simplest route: a plastic or engineered-wood pallet sidesteps treatment and marking altogether.
Shipping out of Chicago
Most destination countries apply ISPM 15 too, so a container leaving Chicagoland for overseas generally needs treated, marked wood packaging. The approved treatments and the exact form of the mark are set out in the standard's annexes; check the IPPC text and your destination's import rules rather than relying on a summary. When you ask for heat-treated pallets through this site, say so in the pallet type and the seller will confirm what their stamp shows.