The wall
The wall symbolizes borders and boundaries, like the border between men and women that the citizens impose on themselves, or like the border between Fiona's private understanding of her worth, versus the external reality, where she can neither communicate her self worth nor defend it. The wall is a symbol for being on the "wrong side" of fate, so that this novel is about the victims of actual disenfranchisement and systematic injustice.
The mark as a symbol
Fiona has a tangible reminder that she is different than others, because she has the forbidden mark on her hand, which she covers up, so only she knows about it. Therefore, the mark signifies the way her character works to understand her worth, by remembering her private knowledge that her point of view is inherently valuable and risky to the damaged community she is exposed to.
The underground
Under the earth, one would think it's solid dirt all the way down, but Arrin teaches Fiona about a realm beneath the daily order of their confusing, frustrating life. Underground, they have privacy where Arrin can invest in Fiona. Therefore, the earth and the underground are symbols for true community which comes from the intimate exchange of vital information and trust.
Bowen as a symbol
Why would Fiona ever want to start a family during a time like this? Yet, when she meets Bowen, she is overcome by love for him, and she wants to start a family with him. For a while, it seems this effort might work, but when Bowen is taken away from her on the finish line, the character becomes a symbol of the desire for life that helps her survive. She must depend on a hopeful future even when there is no evidence that one is coming.
The abandoned building motif
The abandoned house in the beginning is similar thematically to the empty house in the end. These are reminders that the society that mankind built is not permanent necessarily, nor did it always exist. This symbolizes the broken nature of human society, which is exactly what the novel explores in its complex picture of social corruption.
