Jim is right in that if your holster has a snap retention, you have to use it.
However, holster restrictions for USPSA are division dependent. Single Stack (1911) and Production (Basically stock, double or da/sa) divisions require open top holsters just like you would find in any store. Those two divisions cannot use race holsters. However, Limited, Limited-10, Revolver and Open can use race type holsters that basically hold the gun by the trigger guard and/or muzzle and are locked in place.