Due to the phonetic shortening from asexual to ace, ace playing cards are sometimes used to represent asexuality. The ace of hearts and ace of spades are used to symbolize romantic asexuality and aromantic asexuality respectively.
What asexuality mean?
Asexual is the lack of sexual attraction to others, or a low interest in sexual activity. It is common for asexual people to have romantic (but not sexual) attractions to others. They may identify as hetero-, homo-, bi-, or pan-romantic. Others are aromantic — that is, they feel no romantic attraction to other people.