Usb cables that come with iMac 2015

This might be a silly question but in 2015 I bought my iMac and it came with a cable that looks like a male to female usb cable. However the female end is either not a usb or is a special type of usb. The only difference from a usb is that it has a divit that won’t Let a regular usb fit in to it. I’ve looked at tons of pictures on web pages but can’t match it to any usb configuration. I’m stumped.