The Sex field on the main People panel is one of the first fields entered after a person’s name. It is intended to record the official or legal sex of the individual.
Gender, while often aligned with sex, is not the same and can be recorded separately. The Gender field is on the Main tab of the Current Person Other Detail dialog (click the … button next to the name fields). A list of frequently used gender identities is provided, but you can enter any value.
See also Pronouns.