The Selective Service Act or Selective Draft Act (P.L. 65-12, 40 Stat. 76) was passed by the Congress of the United States on May 18, 1917. It was envisioned in December 1916 and brought to President Woodrow Wilson's attention shortly after the break in relations with Germany in February 1917. The Act itself was drafted by then-Captain (later Brigadier General) Hugh Johnson after the United States entered World War I by declaring war on Germany. It authorized the federal government to raise a national army numbering in the hundreds of thousands with which to fight a modern war. The Act was canceled with the end of the war on November, 1918.
At the time of World War I, the U.S. Army was tiny compared with the mobilized armies of the European powers. As late as 1914, the federal army was under 100,000, while the National Guard (the organized militias of the states) numbered around 120,000. The National Defense Act of 1916 authorized the growth of the army to 165,000 and the National Guard to 450,000 by 1921, but by 1917 the federal army had only expanded to around 121,000, with the National Guard numbering 181,000.
By 1916, it had become clear that any participation by the United States in the conflict in Europe would require a far larger army. While President Wilson at first wished to use only volunteers to supply the troops needed to fight, it soon became clear that this would be impossible. Indeed, three weeks after war was declared, only 97,000 had volunteered for service. Wilson accepted the recommendation by Secretary of War Newton D. Baker for a draft.
General Enoch H. Crowder, the Judge Advocate General, when asked for his thoughts on the proposal, indicated his displeasure. Ironically it came to be not only that Crowder, with the assistance of Captain Hugh Johnson and others, guided the bill through Congress but also that he would administer the draft as the Provost Marshal General.
One notable problem that came up in the writing of the bill and its negotiation through Congress was the desire of former President Theodore Roosevelt to assemble a volunteer force to go to Europe. President Wilson and others, including army officers, were reluctant to permit this for a variety of reasons. The final bill contained a compromise provision permitting the president to raise four volunteer divisions, a power Wilson did not exercise.
By the guidelines set down by the Selective Service Act, all males aged 21 to 30 were required to register for military service. At the request of the War Department, Congress amended the law in August 1918 to expand the age range to include all men 18 to 45, and to bar further volunteering. By the end of World War I, some 2 million men volunteered for various branches of the armed services, and some 2.8 million had been drafted.  In fact, more than half of the almost 4.8 million Americans who served in the armed forces were drafted.
Due to the effort to incite patriotic fervor, the World War I draft had a high success rate, with fewer than 350,000 men ”dodging” the draft.
The biggest difference between the draft established by the Selective Service Act of 1917 and the Civil War draft was that a substitute could no longer be hired to fight in a man’s place. In the Civil War, men who did not desire to fight could hire a substitute. However, because it was expensive to hire someone, only very rich people could afford to do so. This resulted in a disproportionately low number of rich men fighting in the war. There was not a specific draft order for the draftee to be put into the service.
However, Section Three of the Selective Service Act of 1917 stated:
No person liable to military service shall hereafter be permitted or allowed to furnish a substitute for such service; nor shall any substitute be received, enlisted, or enrolled in the military service of the United States; and no such person shall be permitted to escape such service or to be discharged therefrom prior to the expiration of his term of service by the payment of money or any other valuable thing whatsoever as consideration his release from military service or liability there to.
During World War I there were three registrations. http://www.archives.gov/genealogy/military/ww1/draft-registration/
After the signing of the armistice of November 11, 1918, the activities of the Selective Service System were rapidly curtailed. On March 31, 1919, all local, district, and medical advisory boards were closed, and on May 21, 1919, the last state headquarters closed operations. The Provost Marshal General was relieved from duty on July 15, 1919, thereby finally terminating the activities of the Selective Service System of World War I.
Conscription was by class. The first candidates were to be drawn from Class I. Members of each class below Class I were available only if the pool of all available and potential candidates in the class above it were exhausted.
|Class||Categories (May, 1917 - July, 1919)|
|I. Eligible and liable for military service.||Unmarried registrants with no dependents, Married registrants with independent spouse and / or one or more dependent children over 16 with sufficient family income if drafted.|
|II. Temporarily deferred, but available for military service.||Married registrants with dependent spouse and / or dependent children under 16 with sufficient family income if drafted.|
|III. Exempted, but available for military service.||Local officials, Registrants who provide sole family income for dependent parents and / or siblings under 16, Registrants employed in agricultural labor or industrial enterprises essential to the war effort.|
|IV. Exempted due to extreme hardship.||Married registrants with dependent spouse and / or children with insufficient family income if drafted, Registrants with deceased spouse who provide sole family income for dependent children under 16, Registrants with deceased parents who provide sole family income for dependent siblings under 16.|
|V. Ineligible for military service.||State or Federal officials, Members of the clergy, Registrants who were deemed either medically disabled or "morally unfit" for military service, Enemy aliens.|