Here we can describe `person/@id` as element ID and `family-member/@ref` as a reference to an ID so our XML tools can link these together.
Also note three more elements from different namespaces: `@sec:checksum` could be some kind of technical information about the record, `fin:credit-rating` is added by the finanical module. The `@last-change` is defined as datetime so as we read it with other XML tools we'll get it as datetime type.
The next one is a tag in Russian language that describes something related to Russia; XML can use the whole Unicode in tag and attribute names.
Also, XML names are globally unique by design so there's no clash between all the different pieces and the tools can easily be configured to ignore parts they don't understand or work as a glue between different areas.
We can still efficiently validate the syntax the whole piece or parts of it as we see fit.