Why are there multiples of the same Length Units

[KevinMurphy]

I was looking into the length units and noticed mile and miUS. Are these the same because it says that a miUS is 5280 ft?

Also, there are two abbreviations for inch: in and inch.

Are these made just for personal preference?

[Tim Wessman]

You have miUS, ftUS and inUS because you have something called a "us survey foot" and by extension a "us survey mile".

Primarily, this is of importance (and quite extremely so) when working with geodesic conversions between different coordinate systems.

https://en.wikipedia.org/wiki/Mile#U.S._survey_mile

"in" vs "inch" is due to the CAS already having a definition that was incompatible with the HP convention... unfortunate, but was made as clean as possible.