Examples
The need for authenticity or realism might require someone of a particular race, sex or age for acting roles (for example, a black man to play the part of Othello) or modelling jobs.
Considerations of privacy or decency might require a public changing room or lavatory attendant to be of the same sex as those using the facilities.
An organisation for deaf people might legitimately employ a deaf person who uses British Sign Language to work as a counsellor to other deaf people whose first or preferred language is BSL.
Unemployed Muslim women might not take advantage of the services of an outreach worker to help them find employment if they were provided by a man.
A counsellor working with victims of rape might have to be a woman and not a transsexual person, even if she has a Gender Recognition Certificate, in order to avoid causing them further distress.