Explain the purpose for registering with the Selective Service System. Is this action a duty or a responsibility?

when needed some men would betold to enter the military after doing activities such as fisical and mental activities! It is a duty

The purpose for registering with the Selective Service upon turning 18 is because when in crisis men would be called upon to enter the military after doing a mental, physical, and moral fitness check. Registering for the Selective Service is a duty.