From Wikipedia:

Specialist (abbreviated "SPC") is one section of the fourth enlisted rank in the U.S. Army, just above Private First Class and below Corporal. It shares the same enlisted pay grade as the Corporal. Unlike Corporals, Specialists are not considered junior non-commissioned officers (NCO).

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In the U.S. Army Corporal (CPL) is preceded by the first three forms of Private and the rank of Specialist. A Corporal ranks above a Specialist and below a Sergeant, but shares the same pay grade (E-4) as a Specialist. Unlike a Specialist, however, a Corporal is a non-commissioned officer and may direct the activities of other soldiers, including Specialists. A promotion from Specialist to Corporal is a lateral appointment; an increase in rank, but not in pay grade.