a human-readable variable label,
an optional unit of measurement,
a concept URI identifying the meaning of the variable
(formerly called definition),
an optional namespace used for value-level URI expansion,
optional labelled values (where supported).
The concept attribute is a general semantic anchor and may refer to:
a measure or dimension concept (SDMX-style), a property IRI (e.g. Wikibase),
or any URI that defines or describes the variable.
defined() vectors preserve their metadata during subsetting, printing,
summarizing, comparisons, and many tidyverse operations. They integrate
smoothly with dataset_df() objects and can be safely flattened via
as.data.frame(), as_tibble(), or coercion helpers such as
as_numeric() and as_character().