I'm pretty sure it's Breq/JoT-as-narrator who says at one point that there's a sea of things other cultures would consider gender markers (body shapes, clothing, jewellery, makeup, hair arrangements). Among Radchaai these are configured in so many different combinations that the decoding other cultures would do wouldn't work anyway (although obviously in the book, other cultures go on body shape). That makes (even more) sense to me of how much Breq struggles with remembering which cues are supposed to be relevant elsewhere.