What does 6% acidity taste like? A friend and I have been doing research on a recipe and he encountered a statement to the effect that the result should be around "6% acidity." I am not familiar with what this would mean in the context of food, and need help parsing what this phrase means and how to tell when a resource is using the taste sensation, as opposed to the pH, sense of the word.

To my understanding, acidity in coffee (as an example) is not actual acidity; "acidity" refers to [the] flavor note, not to the actual acid content; coffee is relatively low in acid. Its pH averages around 5.0 - 5.1(src). 

That said, pH:dilution of vinegar is directly regulated based on it's percent acidity.


If a cooking resource refers to a percent of acidity, is this a measure somehow related to the pH of the food? Or is it possible the resource is making the same "flavor note" comparison (i.e. where coffee turns between having a bright tongue, or not)
Either way, how would I go about trying to prepare some solution that would replicate the taste/sensation of "6% acidity"? 

I am roughly imagining some quantity x of [consumable acid, e.g. vinegar] and y of water; would that approximate the taste/sensation?
In wine-making, at least, acidity is a function of three different types of acid.  Because those types of acid matter to the final product, wine making supply stores and websites provide several inexpensive ways of determining acidity.  Two described here are an acid titration kit and a pH meter.  When making wine last year I used an acid titration kit and it couldn't have been simpler.  You take a sample of your liquid, the indicator solution, and a reactor solution.  Based on how much of the reactor you have to add to change the color of the original sample you can get your acidity in a percent.
In my experience with the wine we noticed the acidity in our grape juice (that was on its way to becoming wine) more by the burning sensation on our hands where we touched the grapes as we crushed them or in our mouths when we drank - our solution started out seriously acidic.  It was hard to taste a difference between too acidic and just right.
If you have a liquid recipe and want to get the acidity exactly right, consider using acid titration.