Subtitles only capture what is spoken on screen. They are straight transcriptions or translations of the dialogue.
Closed captions aim to capture all events in the audio track besides dialogue. They may well indicate gunshots, screaming, a song being played etc. "Closed" means they can be turned off.
17% of Americans report some sort of hearing impairment, which amounts to over 50 million people. That percentage is more or less reflected worldwide. Closed captions are mainly designed for these viewers. BUT – it’s not just about physiological hearing issues. Your audience will often watch films on a laptop in a noisy cafe, or a tablet on a commuter train, or a TV in a bar — or simply at home in the kitchen with the sound of cooking and kids. Closed Captions allow consumers in all these scenarios to enjoy your content comfortably.