This is a story about the broken layers of unspoken assumptions that sometimes govern social opinion. For instance, Arrin knows from experience what Fiona could never have guessed from her state of innocence, that even her gender, something as inconsequential as her being a girl, is a huge detriment on the "wrong side" of the wall. Therefore, the right side of the wall would be one where people were in the enfranchising business, instead of looking for reasons to abuse each other.
In the end, the imagined scarcity of life ends up being the most brutal, violent lie among the society. Fiona is pitted against Arrin, and Arrin is promised with no assurance but her faith in evil people, that if she kills Fiona, they will spare her life. This is a portrait of the way society sometimes pits women against each other by suggesting that there is some sort of scarcity. There is no scarcity behind this unnecessary, violent competition.
The sign on Fiona's sign is a kind of marker, literally making her a "marked" character from a literary point of view. This special sign serves as a secret reminder about herself in the maelstrom of assumptions that she endures from her external life. She is on the "wrong side" of the wall in her life too, because she privately knows that her life is valuable, but her community has universally abandoned the belief that human life is inherently valuable.
