Earlier quoted context omitted.
US slavery was legal, and it took a civil war to change that. The right of people to self-determination is actually a quite important concept in the international law. https://en.wikipedia.org/wiki/Self-determination
The US made a war against separatists, in order to ensure the civil rights: the losers wanted a self-determination involving the "right" of slavery. So "self-determination" for evil/egoistic/xenophobic causes is not reasonable at all. In Catalan region the population is split in two. It is not a "revolution against the central government", but for the power inside the region.
I think they are trying to determine how the population is split by the voting.
If it's a large enough population that doesn't want to live under the Spanish rule, they shouldn't. And if there's a minority that does want to live under the Spanish rule, they still can, just move to Spain.