Sorry for the delay in answering your question.
As I understand it, the answer is actually that series: 16 is rather arbitrary, and the majority of assertions simply stick with series: 16. It does not correlate to an Ubuntu release, for example, but could be used to define how a definition or namespace might be compatible with other series, such as 18 being compatible with 18 and backwards compatible with 16.
I’ve updated our Glossary to hopefully explain this better, and I’ll link to this when mentioning series:: Glossary