In Spinelli's Stargirl, what is the purpose for the "Card Game" that Stargirl and Leo play?

In chapter 21,
Leo discovers thatmakes her own greeting cards. When she finds someone who needs a gift or a
card, she anonymously gives them what they need. The way Stargirl finds out what people may
enjoy for a gift or as a title of a card is through newspaper articles and public bulletin
boards. When Leo gets to view some of her handmade cards, he notices that they are simple and
hand drawn with stick figures. Although simple and not very professional-looking, each card is
personal and heartfelt. This gives them more value than a card bought from the store.

Leo also discovers a game that Stargirl plays while in public. She follows and watches
someone for fifteen minutes and then imagines what type of card she would send them. Leo points
to someone and asks what type of card would...