How is the linkage done between attribute and subject variable?

[KatrinHeinze]

https://vscode.dev/github/sdmx-twg/wp-sdmx-dpm/blob/main/docs/models-relationships/02_data_definition/02_high_level_mapping_summary.md#L17-L18

This is done via RelatedConcept entity.
“variable_attribute” links Fact Variable or Key Variable (at the source of Relation) to its
Attribute Variable (at the target of Relation)."