If you are a male U.S. citizen or immigrant to the United States who took up residency in the country before your 26th birthday then you are required to register with the Selective Service. Failure to register can disqualify you from receiving a number of government benefits, including student financial aid, or receiving a driver license in some states.
The Selective Service System is a government agency that registers American males between the ages of 18 and 25 in case the country ever reinstates a military draft.
For transgender individuals, those who were assigned the sex of male at birth (i.e., on a birth certificate) but identify as female are required to register. Those who were assigned the sex of female at birth but identify as male are not required to register.