For enlistment purposes, the military breaks education into three overall categories: Tier 1, Tier 2, and Tier 3. The vast majority (over 90 percent) of all enlistments are from the Tier 1 category.
Applicants in Tier I have a high school diploma, or at least 15 college credits. This means a high school diploma, not a GED. Depending on state law, completion of high school by home study may or may not be considered equivelant to a high school diploma.
Tier II includes GEDs, home study (in some states), Certificate of Attendance, Alternative/Continuation High School, Correspondence School Diplomas, and Occupational Program Certificate (Vo/Tech). The services limit the number of Tier II candidates it will allow to enlist each year.
In the Air Force, the limit is less than one percent each year. In such cases, the applicant must score a minimum of 50 on the AFQT to qualify (Note: The "AFQT" is the overall ASVAB score).
The Army will allow up to 10 percent each year to be Tier II candidates, but they must score a minimum of 50 on the AFQT.
The Marines will only allow about 5 percent each year to be Tier II, and the Navy about 10 percent. Like the Army and Air Force, Tier II recruits must score a minimum of 50 on the AFQT to qualify.
The Coast Guard only accepts Tier 2 candidates if they have prior military service, and even then requires them to score higher on the AFQT (50 for prior Coast Guard Service, 65 for prior service in other branches).
Individuals who are not attending high school and are neither high school graduates nor alternative credential holders. The services almost never accept a Tier 3 candidate for enlistment. If you fall into this category, your best bet is to get at least 15 college credits, so that you will be qualified as Tier I.