I'm rather familiar with condition levels of carrying for pistols, but I have a vague grasp of retention levels on holsters. As soon as I think I get it, I see something that screws with the list.
The "retention level" of a holster is just the number of locking devices that have to be disengaged before the gun can be drawn.
What creates some confusion is that Blackhawk misnames their holsters. Their CQC Serpa line are labelled as "level 2", but in fact they're level 1. You have to hit one button, then pull.
I'm not aware of any other manufacturers doing that. With Safariland, for example, a level 3 holster has three devices: a thumb brake, a middle finger release and an ejection port lock that is unlocked when you rock the gun backward.