The distinction turns on design intent and primary function. A machining center is characterized by its automatic tool-changing system and ability to perform multiple different operations (milling, drilling, boring) in a single setup. A machine designed principally for drilling — even with CNC control — that lacks an ATC and is not capable of milling operations is properly classified as a drilling machine under 8459.21. When a machine straddles both descriptions, the principal function at the time of importation governs, and a CBP binding ruling can provide certainty.

CBP generally expects the commercial invoice to state the machine's condition explicitly, along with the year of manufacture and any refurbishment details. Supporting documents such as a prior owner's maintenance log, a refurbisher's certification, or a surveyor's condition report strengthen the declaration. Importers should also ensure the customs value reflects the actual transaction value for a used machine, as CBP may compare declared values against market benchmarks for known models.