Why it gets attention
Dunnage is often cut on site from whatever timber is at hand, which makes it more likely than a manufactured pallet to be untreated or to carry bark. ISPM 15 singles it out for that reason.
Marking
ISPM 15 says shippers should make sure all dunnage used to secure or support goods is treated and shows the mark, clearly and legibly. Because dunnage is often cut to length only at loading, the standard notes that marking it legibly needs special care, and it says small pieces of wood that do not carry all the required elements of the mark should not be used as dunnage.
A narrow exception
Timber or lumber shipments may be braced with dunnage of the same type and quality of wood that meets the same requirements as the consignment. ISPM 15 says such dunnage may be treated as part of the consignment rather than as wood packaging material.
Euro pallets as dunnage
Broken euro pallets are sometimes cut up for bracing. If the pieces cross a border again, each one needs to show it came from treated wood. Cut pieces that lose their mark are a common reason for problems at inspection. Recycling broken pallets is usually the better route.