The character is a subject of sex-change.

In real life sex change is a term often used for sex change in humans, that is all medical procedures transgender people may pursue, or specifically to sex reassignment surgery, which usually refers to genital surgery only.

In visual novels, sex change is rarely realistic and usually involves futuristic technology or some sort of magic. Or alternatively the developers doesn't care much for explanations and the character just wakes up one day with his/her gender swapped.

The trait should only be used if the change happens during the course of the game and not before it even starts.