According to this report from the AP:
Law enforcement officials around South Carolina are preparing for the new statewide seat belt law that goes into effect Friday.
The new law is a primary one, which means law enforcement officers may pull a vehicle over if they suspect anyone inside is not wearing a seat belt. Under South Carolina's current law, which is secondary, officers may issue a ticket for a driver or occupant not wearing a seat belt only if they have stopped the vehicle for another violation.
But the new law also prohibits officers from setting up checkpoints specifically to check seat belt or from writing tickets for seat belt infractions at other public safety checkpoints.
In the criminal law context this means that police have yet another "valid reason" to pull over a vehicle if they have a hunch some other criminal activity is afoot.