Tethered capture focus buttons

Hi
I have a Canon 6D which I have been using with the Tethered Capture feature in Lightroom CC, which is a fabulous tool. I could probably find this out through experimentation, but can anyone tell me what the difference is between the focus buttons. I appreciate they are three stages of focus adjustment, but if a single arrow is one focus unit, how many focus units does the 2-arrow button represent and how many focus units does the 3-arrow button represent?
Has anyone asked that question before!