liter(metric) gallon(customary)
One US gallon of producer's milk (not skimmed, semi-skimmed, homogenised etc), weighs 10.1 pounds-force in customary measure and 44.5 newton in metric measure.
It is a customary unit.
The opposing systems are imperial and metric, not customary. But, meters are metric.
The pound is NOT a metric unit.
It is customary.
It can be more or less. An once (customary) is bigger than a gram (metric) but a pound (customary) is smaller than a kilogram (metric).
Metric.
Metric
Metric.
It is a customary unit.
customary