[masklinn]

Basically, US Customary is english units pre Imperial standardisation.

Then in 1959 the major non-metric countries (I want to say US, UK, SA, AU and NZ?) unified their units under the "International Yards and Pounds". This metrified the definitions of various non-metric units.

So for all the units with an "international" version, "US customary" and "imperial" are now the same thing (though the pre-international versions might still be used in some contexts e.g. US surveyor units are, in fact, pre-international US customary units), and there are units which were not unified, most notably units of volume which remain different in the US and in imperial dominions.