space invaders doesn't need any sample set anymore since 114u2. So it's unneeded and can be removed.
Although, Mamedevs forgot to remove relations from other sets which kept invaders as sample-parent. That's where lots of warnings come from when you get the data directly from mame. So...you have to copy the invaders sample set for each of its sample-clones.

And to answer your original question: It's sets+unneeded check which toggles this. I still don't know why people always think of 'set' as a set of roms....
A set can be a collection of roms and/or samples and/or chds....so each of them alone is possible.


Roman Scherzer