For Mathematica 12.2.
These two are equivalent
Sort@Keys[QuantityUnits`Private`$UnitReplacementRules] ==
Sort@QuantityUnits`$UnitList
(*True*)
There are also
QuantityUnits`$UnitAlternateStandardNameReplacements
contains something like Celsius
which is not contained in $UnitList
. Most of these units are readily usable in Quantity
except few cases need internet connection for interpretation.
There are also UnitShortName
information in QuantityUnits`$UnitTable
shortUnitAssoc = QuantityUnits`$UnitTable // Query[;; , "UnitShortName"] //
Select[# =!= None &];
which contains "Milligrams" -> "mg"
, "Kilograms" -> "kg"
, "Meters" -> "m"
etc. Some of these short unit are readily usable like Quantity[1,"mg"]
.