OptionaldisplayProperty subjects only: one value per matched property, a list, enumerated
or table value as its joined display text, where values holds each
member (#5475). The set checks (unique, aggregate, compare) read
a property as ONE value, so they use these; element checks match members.
At least one value, after String(v).trim(), is non-empty (plan §4.2).
OptionalunitQuantity subjects only — the stored unit label, when known.
OptionalvalueProperty and quantity subjects only, aligned with values: the factor
that turns each value into SI base units, from the same unit as
valueUnits; undefined for a value with no unit (#5225).
OptionalvalueProperty and quantity subjects only: the unit each entry of values is
recorded in, index for index (#5300). The value's own explicit Unit
when the file declares one, otherwise the project unit for its measure
type; undefined for a value that has no unit (a label, a count, an
untyped property).
The result
readSubjectreturns for everySubjectkind.