Unified refers to the crest of the thread, and to it's specific radius. Non-unified used a sharp thread crest, which permitted some galling in long term use. Unified was an agreed-to radius that American, Canadian and British thread makers standardized on in the post-war era. The measurement system of feet and inches, miles, gallons, and so on, is Imperial. The only deviation is in liquid measure, the US gallon being smaller than the standard or "imperial" gallon.
Strangely enough there are more variations in the metric system than there are in the Imperial ones. Canada has been metricated for thirty years and I still think in feet and inches, and miles per gallon, and pounds and horsepower......