From your context it is impossible to define what "C" might be, and it is not a generic term. "C" is used as an abbreviation only for Celsius, so it can't be a material, unless you discover a relationship in your text. So "C type parts" or "C parts" is the only way to do it.

This is part of an inventory classification to help companies decide internally where to invest most effort/streamline systems. Based on the Pareto principle that '80% of a company's cost value is provided by 20% of the raw materials'. 'A' items account for a large proportion of overall value within a company but a small percentage of number of items, the reverse applies to 'C' items.