What Does C Look Like On A Tuner How Do You Use The "key" Setting On A Chromatic Tuner For Guitar?

How do you use the "key" setting on a chromatic tuner for guitar? - what does c look like on a tuner

I never really learned music, I took the guitar with tabs in the Internet and could not say that a deal seems as if you had asked me to play a card. I recently received a chromatic tuner, a Seiko SA 500, and has called an important parameter, and I have no idea what it is. The 4-configurations are Bb, F, Eb and C can you tell me what is it?