letter in three sports

'Chicken Soup for the Soul' を読んでいます。

I should tell you a bit about my daughter. She's a tomboy, a beautiful five-foot−nine athlete who letters in three sports each year, frets over the difference between an A- and a B, and was just named a finalis in the Miss Teen California contest.

"letters in three sports"　の個所が分かりません。　辞書で調べます。 "letters"

・Macmillan Dictionary: to earn a prize in high school for excellent performance in sports
・Cambridge English Dictionary: If a student letters in a particular sport, they are given a prize for achievement in sport in the shape of the first letter of the name of their high school or university, which is often worn on a special jacket or sweater: She lettered in five varsity sports at Berkeley last year.
・Vocabulary: an award earned by participation in a school sport: he won letters in three sports”

この "letter" は動詞も名詞の用法もありますが、いずれにせよスポーツに限られている様です。