“All male U.S. citizens and immigrant non-citizens who are between the ages of 18 and 25 are required by law to have registered within 30 days of their 18th birthdays [to The Selective Service] […] The Selective Service System is a contingency mechanism in the event conscription becomes necessary.”
They may be referring to https://en.wikipedia.org/wiki/Selective_Service_System
“All male U.S. citizens and immigrant non-citizens who are between the ages of 18 and 25 are required by law to have registered within 30 days of their 18th birthdays [to The Selective Service] […] The Selective Service System is a contingency mechanism in the event conscription becomes necessary.”