Seems like this link has the descriptions. And, all of this is in reference to the "scary Black" AR type rifles.
Featureless is what people are buying in CA, no thumbhole stock, no pistol grip, etc...... so what you can legally buy in CA right now, "scary Black" rifle types and any other legal semi-auto's with detachable magazines like a ruger 10-22 or a mini-14
compliant is a rifle with at least one of those features, BUT a fixed magazine
Registered assault rifles have "features" and a detachable magazine and that window is closed, either they are already on the registered list, or they are not legal
SO, in the You Tube clip he is saying you can put your standard magazines on a Featureless, but not on a compliant ( as those still may not have a detachable magazine at all) or on the regisered assault weapons, as those also must stick to the way they were made and registered.
https://caligunner.com/california-compliant-featureless-rifle/A Featureless Rifle is the term used to describe a semi-automatic rifle that does not have any of the following:
A pistol grip that protrudes conspicuously beneath the action of the weapon
A thumbhole stock
A folding or telescoping stock
A grenade launcher or flare launcher
A flash suppressor
A forward pistol grip
So even if you have JUST ONE of these features on your semi-automatic centerfire rifle you own what California considers to be an “assault weapon” unless your magazine is fixed.
Is Going Featureless the Only Way to Be Compliant?
No! There is another option that allows you to keep your features (pistol grip, folding/telescoping stock, forward grip and flash suppressor) while still being completely legal. This is the fixed or magazine lock option.
Fixed or Locked Magazine Option
This option allows the use of features on your AR-15 by fixing or locking the 10 round magazine in your rifle while the lower is attached to the upper receiver. This is often done with a device like the Juggernaut Tactical Hellfighter Kit available at Brownells. With a device like this you can use all the “evil features” described at the beginning of the article and still be legal. This is a great option for those that can’t get over the looks of a featureless rifle.
There are pictures and much other useful information in that article, options, suggestions, etc....