Auxiliary plant (840410) refers to functional units that serve the boiler system — such as economizers, superheaters, and feedwater heaters — while 840490 covers discrete parts of those auxiliary units or of the condensers. A complete, self-contained auxiliary assembly generally falls under 840410; individual replacement components fall under 840490.

Yes. Condensers specifically designed for steam or other vapor power units are classified under 840420. General-purpose industrial heat exchangers not dedicated to vapor power service are typically classified elsewhere in the tariff schedule, so the intended application and design specification are the key classification factors.