The classification follows the physical state of the gas at the time of importation. LNG and LPG cargoes transported in cryogenic or pressurized vessels are classified under the liquefied subheadings (271111–271119). Pipeline natural gas arriving in gaseous form is classified under 271121. The same molecule — for example, propane — falls under different subheadings depending on whether it arrives liquefied or as a gas.

Liquefied mixtures of ethylene, propylene, butylene, and butadiene fall under subheading 271114 when those olefins are the predominant components. If the mixture is predominantly a saturated hydrocarbon such as propane or butane, it may instead fall under 271112 or 271113. Detailed compositional analysis (GC chromatography) is typically required to support the correct subheading, particularly for mixed NGL streams.