Hi Ken and welcome. Compliant refers to some States that put restrictions on certain firearm configurations like, magazine capacity, threaded barrels, pistol grips, muzzle brakes or flash hiders etc. The standard model comes with (3) 23rd magazines and threaded barrel whereas this compliant version comes with (3) 10rd magazines and a non-threaded barrel. It will also accept the 23rd magazines but no option to add a muzzle brake of suppressor since the barrel is not threaded. Cheers and stay safe!

Hi Ken, there are states that infringe upon the rights of it's citizens and ban certain features, like collapsible stocks, flash hiders, threaded barrels, and magazines over 10 rounds. Compliant means this gun lacks some features which allows it to be sold in some, but not all, restricted states.
In this case, the gun has a non-threaded barrel and is sold with (3) 10rd magazines, instead of the usual 30rd. (it will accept higher capacity mags)
Stay safe & have a great day!