Auxiliary plant refers to complete functional units—such as economizers, feedwater heaters, or soot blowers—that serve the boiler system but are not integral structural components of the boiler itself. Parts, by contrast, are components that make up the boiler or auxiliary unit. The distinction matters because auxiliary plant and parts carry separate SITC headings and may attract different duty rates.

Potentially yes. When auxiliary equipment arrives as part of a complete industrial plant under a single contract, importers sometimes explore composite goods or sets classification rules. However, each article is generally classified separately unless it meets the strict criteria for an unassembled machine set. A licensed customs broker should review the contract structure and shipping documentation before entry.