depends on where the gun is being used (division rules for holster placment and type)

download the latest USPSA rule book and check out the appendixes for more divsion specific holster restrictions.

5.2.7 Competitors must not be permitted to commence a course of fire wearing:
5.2.7.1 A shoulder holster or “tie-down” rig (visible or otherwise),
except as specified in Rule 5.2.8,
5.2.7.2 A holster with the heel of the butt of the handgun below the top
of the belt, except as specified in Appendix D, or otherwise indicated
in Rule 5.2.8.
5.2.7.3 A holster with the muzzle of the handgun pointing further than
3 feet from the competitor’s feet while standing relaxed,
5.2.7.4 A holster which does not completely prevent access to, or activation
of, the trigger of the handgun while holstered.