I’m not here for the unnecessary arguments, but I’m of royal descent myself, so I want to share my insight. This is based on my own understanding of my lineage’s history without looking up about royal systems. So I welcome anyone to discuss about it.
Royalty is part of nobility
You’re technically right, but that’s an oversimplification. Royalty and nobility differ in terms of bloodline status, hierarchy, duties, roles, and privileges, etc.
Basically, Royalty are not simply nobles, but they can be. While nobles can’t simply be Royalty unless they have blood, or start their own royal house due to circumstances where the initial royal line ends. At this point you can even say outsiders and commoners can be royalty.
If you want to talk about bloodlines, many monarchies, are tracked through the male line. Especially in patrilineal succession systems. Typically, direct male descendants of a crowned monarch are considered royal: sons, grandsons, and so on. If a monarch has no sons, the throne may pass to a brother or nephew, still preserving the male royal lineage.
In systems where female succession is not recognized or limited, royal daughters may retain noble titles, but don’t pass royal status to their children if they marry outside the royal family. Their descendants may still be of noble birth, but not royal. This is where one of the line is drawn.
That said, fiction often takes creative liberties. A character might be called a “rightful heir” even if descended through the female line especially if they’re the last of the bloodline.
In real world cases, like someone mentioned, when a royal line ends or becomes ineligible, another royal house or branch with shared ancestry, such as a cousin line, may take over. This happens especially in monarchies that prioritize bloodline purity and male succession.
What that person mentioned was something similar to what had happened for my lineage’s history as well.
I saw you’ve questioned where the line is drawn and what if the royal lineage ends. So I hope this answers. Truth be told, I’m not sure if a line is even drawn for the parts where the descendants still fit the requirements or I could have missed out some of my own lineage’s history.
Otherwise, it’s usually just a matter of whether they will be relevant.