I am asking on behalf of my friend.

My friend doesn't have a gender (in other words they are gender-neutral) and on their new passport their gender is represented as X (indeterminate/unspecified). Previously booking a flight wasn't an issue because theirs previous passports stated they are a woman.

The problem is every time you book a flight you need to choose from 2 genders. Now they will be travelling from a Sweden to Russia but the question applies to their future flights too, anywhere in the world.

What gender should choose a person who is not cisgender when booking a flight? Does it depend on a flight operator or flight destination?