[Dark] tag

I just had a very interesting argument discussion on a story about when and where to use the [Dark] tag.
As far as I can tell, it actually has two usages, which are kind of... different.
First, circumstances in which the consequences are dire and the main characters are, by many means, screwed. This is more of a tonal thing, though. If it's a dark terrible atmosphere, it's a [dark] story. That's fine.
But then the description adds that itr should also be used for stories with "unsettling concepts". This is very unhelpful indeed because unsettling is not an objective statement. What I find unsettling isn't going to be what you find unsettling. I hate dolls, bugs, freeform jazz and the miracle of childbirth, but I personally know someone who likes all of these things. Except Freeform jazz.
So, if the tag goes on things that include unsettling concepts, where do we stop? I have a story about a pony cursed to endure strange and horrific cravings late at night. Is that unsettling? Should I tag that? What about my first, and most awful, story, about the love between two stallions, and the questions over their family's acceptance, is that dark?

I honestly want to know if I should tag any, or all, of my fics [dark].