Sometimes a cup is a cup, and sometimes it’s not.
In the U.S. a standard “one cup” measure is eight fluid ounces. That’s what a typical coffee cup will hold. Except that’s not what the typical coffee brewer will deliver. The capacity of most brewers is based on a six fluid ounce cup. Unless, of course, it’s a European brewer [or descended from one] in which case its capacity is often based on a four fluid ounce cup. [?]
Of course, the coffee cups that I favor have a capacity of a pint or more. My favorite holds two. Pints. Or four cups. Or eight cups if you’re European.
Now that’s a cup.