Nearly two years after President Carter ordered renewed registration for the draft, the Selective Service System yesterday set forth its official blueprint for pulling young men into the military in case of war.

Selective Service issued a detailed set of proposed rules to implement the mobilization plan Carter called for after the Soviet invasion of Afghanistan. The new draft regulations should take effect early next year--unless President Reagan decides to abolish draft registration altogether, as he promised to do in last year's campaign.

The White House says a final decision on registration will probably await next month's report from a task force studying military manpower issues.

Reagan yesterday issued a statement saying that a voluntary military is "the best way to meet our military manpower requirements in times of peace." See story, Page A4 The question the Selective Service regulations deal with, however--and the question still facing Reagan--is how to prepare for the manpower requirements of a war.

Throughout the Cold War and Vietnam era, the nation relied on a draft system that required young men to register on their 18th birthday and then take various physical and written tests to determine their classification. When the military needed men, Selective Service inducted those who had not been excused because of medical, professional, family, or philosophical reasons. That system, which ended when the draft was stopped in 1973, was designed to supply a standing peacetime Army.

The new regulations proposed yesterday have a different purpose. The new Selective Service System is designed only to provide mobilization in time of war. The new mechanism, therefore, will work quite differently.

Under the proposed rules, men would have to register at 18. But all registrants (except those with a permanent disability) will automatically be classified "1-H," which is basically a holding pattern, and nothing else happens until the president declares an emergency that requires mobilization.

In that case, under the proposed rules, all registrants would automatically be reclassified "1-A," meaning fit for duty, and those selected for service--under a birthdate lottery-- would immediately receive notices of induction into the armed services. Only at that point would draftees be allowed to claim an exemption. Some draftees would be given only 10 days to make a such a claim.

The Selective Service says these new rules will be easier for the government to implement and less intrusive for draftees. Critics of the system say it make things tougher for men seeking an exemption. "You can't create a file in 10 days," says Thomas Alder, publisher of the Military Law Review. "And there isn't time for people to learn their rights. For a lot of people, this means you show up at the induction station, and if you can't present your best case then and there, you're on the bus."

Alder says the motivation behind the new rules is largely political. "They did it this way so they can keep saying right up until the day war is declared, 'We have only registration, we don't have a draft.' "

One venerable element that is changed in the new draft plan is the traditional induction notice that began "Greetings . . ." The new draft notice is a four-page mailgram that begins "This is your order to report for induction."