The letters correspond to the physical connector. The rectangle one everyone knows is A, B is the square connector usually used with printers and other accessories. C is a small, oval connector, and the first one that can be plugged in “upside down” because it’s symmetrical.
C is now common on phones and tablets and other smaller devices. If the connector is ovalish but even smaller and weirdly shaped, that’s one of the mini/micro USB variants and you need to find the right cable to match.
(If you have a USB C cable and i won’t fit into the oval connector because it’s too big, that’s Lightning made by Apple before C existed and is only found on their devices. You have to get a cable with Lightning on at least one end.)