Being able to distinguish settings and systems may be useful. Forgotten Realms is a D&D setting, but it's for several D&D systems (versions). D&D can generally be played without regard to the Forgotten Realms and still work. But playing 40k as a system without being in the 40k setting is pretty much impossible. Some systems (GURPS, FATE, d20 modern) are super generic and divorced from settings entirely. Others share an underlying base system (PbtA, or Mork Borg) but the setting layers its own systems on top of that base.
In the end, you're going to have a hard time sorting things into a hierarchy, so it's good you're using tags.