I have been told "all apples make cider". I have been asked "what is a cider apple?". Well, the Cider Apple trees that we grow are specifically to produce hard cider. We grow fermentation fruit trees. When a sweet, or dessert, apple is pressed, it makes a lovely sweet cider. However, if it is fermented, the sugar changes to alcohol, and there may not be many complex flavors remaining. The Bittersharp and Bittersweet varieties that we grow are high in tannin, which is a component of hard cider that can't be found elsewhere, or in other (non-cider) apples. Bittersharp apples have high tannin and high acid. Bittersweet apples have high tannin and low acid. The tannin in the apple is very harsh when you bite into it (or it can sneak up on you if there is some sweet that hits your tongue first. So you spit it out!
Cider apples have a lot in common with wine grapes in this regard. You don't see pinot noir grapes in the grocery store, and you probably wouldn't want to make a pie with a cider apple!
The picture is of Kingston Black cider apples in our nursery in August, 2017. The photo is special because we don't normally have apples at all. Our "babies" usually leave home before they produce.