What does it mean to bet on a Field Horse? (ex. 1A)

If you see a race that has a horse numbered 1 and another at 1A, this means if you bet on it to win, then you have two chances for your bet to win the race instead of betting on one horse. Meaning if the race finishes 1A-4-8 then you're a winner. Just like if it finished 1-4-8.