Nullification refers to various surgeries wherin someone may have their genitalia removed but not replaced.
See also - salmacian.
Why would someone not want genitalia?
For some people it causes gender dysphoria to have genitalia of any kind.
Who would get these surgeries?
Typically non-binary people.
Note - These surgeries are fairly uncommon, even among non-binary people.
Art Surgical - Gender Nullification Surgery
Busy Bee - AMAB-to-neutrois GRS
Mozai Care - Nullification
Vice - Why Some Men Choose to Surgically Remove Their Penis
Wikipedia - Penectomy
YouTube: New Odd Times: "I Cut Off My Penis" - The Life of a Nullo