Perfect davidfor, thank you!
That is exactly the clarification I was looking for.
I have used Calibre but I dont really like that app. Instead I use Sigil.
In this case Ill probably forget about "Series", its device-based & harder to access/edit than collections, so doesn't feel worth the effort in my usage. But thank you for the explanation.
It's a shame Kobo doesnt offer a xml tag way to organise, users could add horror, sci-fi or whatever. But thats a discussion for another topic.