Nora has a birthmark on the inside of her wrist. When Patch first observes it he says, "Looks like a scar. Are you suicidal, Nora?" (1.83) Nora quickly corrects him and says it is a birthmark, but she later identifies it as a scar herself. Jules has the exact same mark, also on the inside of his wrist. And what do you know? Patch has a birthmark on his wrist as well, shaped like a splattered raindrop, a convenient image for a fallen angel, dropped from the sky like the rain. He has two scars on his back as well, which form an upside-down V shape.
Birthmarks and scars are imperfections of the skin. A birthmark is an imperfection you're born with; a scar is one that is gained, an indication of something that's happened in the past. In this book, these physical imperfections suggest that there is some sort of internal imperfection going on with the bearer.
The mixing of the terms "scar" and "birthmark" is never quite cleared up in Nora's case, but the imperfection on her wrist is referred to more often as a birthmark, so we'll run with that in our analysis. The birthmark is a sign that she is related to Chauncey, Patch's vassal. She has "bad genes" (23.111), as Patch puts it, meaning Nora was born a player in the dark struggle of Nephilim and fallen angels.
So while Patch's early comment about the birthmark representing a suicide scar is a totally un-cool joke, it does toss out the notion that the mark is linked to life and death, which it is. The birthmark sits on the vein and pulse of Nora's wrist, an indication that it is a fundamental part of her being—just like her status as Chauncey's relative.
Unlike a birthmark, which is ever-present and destined, a scar is gained. Patch gets his back scars from having his wings ripped out because of his transgressions, and they work as reminders of his dark past. They also operate as portals to the past. When Nora touches them, she is sucked back in time to scenes from earlier in Patch's life. The wings may be gone, but through the scars they leave, Nora is able to fly through time.