Medium-dark describes a position within a roaster's own range rather than a universal specification. The source places it beyond a medium roast, with roast character becoming more evident and sometimes with a slight sheen on the bean surface. Those observations are useful clues, not precise measurements.
A medium-dark coffee may suggest caramel, chocolate, spice or a bittersweet finish, with less conspicuous brightness than a lighter interpretation of the same coffee. The result still depends on the green coffee and the way the roast is managed. Longer roasting alone does not guarantee a fuller or better-balanced drink.
Terms such as Full City, Viennese or light French may overlap with this part of the spectrum, but their use differs between businesses. Compare the actual descriptions and ask about the intended brewing application. When testing a new bag, begin with your normal measured recipe and then adjust thoughtfully. The label is most valuable as a conversation starter about flavor, not as a dependable translation between every brand.
The conversation
A good question can make a better brew.