There are many rituals for which we use wine. I would think that wine means a fermented grape product, or at least a fermented fruit product. On what basis do so many people use grape juice instead of wine for rituals like Kiddush? If there's a leniency to allow something similar to wine (viz, grape juice), why not other things that are similar to wine (e.g., fermented pomegranate juice (aka pomegranate wine) or plum wine)?
I know there are some times when we are less particular generally and allow whiskey or even orange juice.
I'm asking about those times when we are particular to have wine, yet we still allow grape juice, albeit sometimes begrudgingly. Why yes to grape juice and no to plum brandy?
I assume there must be some underlying principle that will explain both sides of this.