The first reference of UCUM at https://www.openehr.org/releases/RM/latest/docs/data_types/data_types.html#_dv_quantity_class is more an “inspiration”: “Units were inspired by the Unified Code for Units of Measure (UCUM), developed by Gunther Schadow and Clement J. McDonald of The Regenstrief Institute.”

This should probably be reworded now to say that openEHR uses UCUM.

The second reference of UCUM is clearer: “Stringified units, expressed in UCUM unit syntax, e.g. "kg/m2", “mm[Hg]", "ms-1", "km/h".”

This is where I think that not only it is stated that openEHR uses UCUM (and not some part or “inspiration” of it).
It also seems to imply that the case sensitive version of it is used (which in my view is important to know at least for some of the units). However, I still think it would be good to explicitly say that the case-sensitive version is used?

It could also be recommended to use a UCUM service may also be able to give a print version, e.g. °C instead of CEL or °F instead of [degF]

6.3.1. Units Syntax describes the UCUM syntax in BNF.
I would suggest to consider to remove this and link to UCUM directly where the actual units are defined.

Also, I am not sure how legal the white space in the example given here is : "kg {total}".
From the UCUM specs at http://unitsofmeasure.org/ucum.html:
"White space is not recognized in a a unit term and should generally not occur. UCUM implementations may flag whitespace as an error rather than ignore it. Whitespace is not used as a separator of otherwise ambiguous parts of a unit term.