Hi Janet, the CAF model is the California compliant model. Their California compliant version has a magazine lock release that requires a tool to remove and attach the magazine. Unlike the aftermarket "magazine locking device," the SRM California compliant 1216 magazine lock is manufactured as a part of the shotgun. It is not an aftermarket system. The common question is, "How is this shotgun legal if it's more than 10-rounds?" We are glad you asked, the 1216 has a quad tube magazine that holds four rounds in each tube. After one tube of 4 rounds is dispensed, you must manually index the tube to the next chamber of 4 rounds. Because it's not a continuous 10-rounds and instead separated into four tubes, it complies with California's law.
The STS is the standard version without the magazine lock.
Both models have threaded barrels, which are not banned on shotguns in CA.
Stay safe & have a great day!

Hi Janet and welcome. The CAF is the California compliant model. If you are not in Cali, I recommend the STS (Threaded barrel) over the CAF (non - threaded barrel). The CAF appears to be out of stock anyways. Cheers, stay safe and enjoy!