In a recent workshop someone asked that question saying it seemed like characters only should apply to fiction writing. My answer is always the same: if you see the people in your life as major and minor characters two things happen: 1) it takes some of the emotion out of how you want to include these people in your memoir; and 2) it allows you to see who were the most important people in your life, and who were minor characters that came into your life at a particular time to help or hinder you but then left your life.
Major characters might be parents, partners, mentors, supervisors, teachers, military superiors, or...
If you are writing about your personal life, major characters might be family members, even extended family members. In an event memoir (a memoir about a particular event in your life - moving, a new job (that worked out or did not work out), a marriage, fertility issues that took a long time to resolve (or not), etc.
Minor characters come into our lives to sometimes point us in the right direction (they might be mentors, or counselors or someone who gives you information you did not know...or...), they might want to stop us from doing something (and in stopping us, without knowing it right away, point us in the right direction), they might come through with a grant, loan, scholarship...(or not), etc.
The minor characters may be significant but they do not stay in our lives. Major characters do - even if they are not there physically, they are in our memory as we remember their advice, things they lived by that you adopted in your own life, and were/are a continuing influence.