What warrants the Dead Dove Do Not Eat tag?

My longfic has quite a bit of dark content and I have tagged each aspect of it, but I'm not sure if it has crossed the line and I should also tag it as Dead Dove Do Not Eat. I'm curious what others would consider content that needs that type of tag... And also curious why this tag is used if the individual tags about the dark content are already used. Thank you for any thoughts or insight!

(I am new to posting fanfic and have been out of the fandom space for a couple of decades so I'm just not sure about some of these etiquette things!)