NAICS 324 is the three-digit subsector encompassing all petroleum and coal product manufacturing, while NAICS 3241 is the four-digit industry group that sits one level below. In practice, both codes cover the same set of industries at this level of the hierarchy, but the four-digit code is used when more granular statistical reporting is required.

Import demand is primarily driven by regional refinery capacity constraints, product-quality specifications (e.g., ultra-low-sulfur diesel requirements), seasonal demand peaks, and price arbitrage between domestic and international markets. Disruptions to domestic refinery operations — from hurricanes, maintenance outages, or feedstock shortages — can sharply increase short-term import volumes.