This is a lot of fun to refer to if you are interested in astrology and/or numerology. It is derived from an ancient way of using playing cards where a deck of cards represented a calendar: 52 cards = 52 weeks. 4 suits = 4 seasons. 12 court cards = 12 months. And so on. There is a card for your day of birth. And you can calculate the composite card for your various relationships.
Note that Robert Lee Camp, author of the more well-known Love Cards and Cards of Destiny books, studied under Sharon Jeffers. I prefer Jeffers’ more intuitive approach.