When shopping for a holster, you will often find yourself confronted with the terms Level I, Level II etc. in relation to the retention properties of your selection. Here is a quick explanation of these levels to help you make your selection.
It's at this level where there is the most confusion with holsters. Most instructors and holster manufactures consider the good old-fashioned plain Jane holster that has no snaps, straps, flaps, buttons, breaks, or other devices to hold the handgun in as a Level I. The pistol is held into these holsters by the gravitational pull of the earth as it rotates and the friction that the sides play on the frame of the gun. These holsters are usually the cheapest, the most easy to use, and the fastest. A handgun in a L1 can be deployed in a second or less by someone used to it. Unfortunately, if you get in a tussle or even if you get up fast from an awkward position, this type of holster can leave your gun rattling on the floor.
For most people this means a thumbreak holster in which some device, usually a strap over the hammer but in some cases a lever or flap, has to be moved or snapped to get to the handgun to leave the holster. These are the minimum safe holsters for law enforcement, military, and security personnel. While the strap, flap, or lever will usually keep the gun in place if the wearer falls or stumbles, it can still be moved very fast and the handgun deployed. They take a little getting used to and its advised that the user practice drawing with an unloaded firearm from one of these holsters several hundred times to get used to it.
Occasionally holster makers downgrade this level to a Level I, making a friction holster a Level 0, that is where the confusion comes in mentioned above.
For those who require more than the force of gravity, the friction of the holster sides, and a lever or strap to hold their gun in, they reach for a Level III design. These designs are trick holsters that incorporate all of the above methods while adding the need to push, pull, or rotate the gun in a certain way before the holster releases its hold. These holsters are rather expensive due to the craftsmanship involved, but can be a lifesaver in the field. In most cases, it's almost impossible for an assailant to remove one of these from the holster of another person, even if they are unconscious.
The Blackhawk CQC is an example of the Level III holster with muliple actions needed to operate. Photo by Frank Borelli, Defense Review
The downside of these is that they require a ton of training to develop the muscle memory to carry and use these holsters effectively. They are also bulky, leading to the primary users being law enforcement on patrol duties or those who open carry with a proper duty belt. For those who still need more retention, such as in the corrections industry, there are Level 4 and even 5 holsters that further add even more tricks to the design.
For most, the Level II is about as effective a holster that is needed. It provides more positive control on the handgun that gravity alone while not having all of the doo-dads, expense, and training required of the Level III.
But of course, to each their own, just make sure whatever you choose that you practice with it and by all means, carry it as much as possible.