What was the significance of the selective service act?

In its original form, it mandated the registration of all US males between ages 21 to 30 and authorised the President to call up or draft suffiecient men to establish four infantry divisions. It was later modified to register all males between 18 and 45 and 2.8 million men were drafted. Half of all WWI troops were drafted.

The selective service instead of draft, referred to the provision that deferments were awarded in what was supposed to be a logical way. The idea was draft men who could be taken into the service with the least disruption to the war effort .