3 eSIM Support
The iPhone XS, iPhone XS Max and iPhone XR all sport two SIM cards — an actual physical SIM card tray and Apple’s proprietary eSIM technology. iOS 12.1 basically enables the embedded eSIM in newer iPhones.
Of course, you can’t use Dual SIM, Dual Standby quite yet. You’ll need to wait for your particular carrier to introduce support on their end. But that’s likely to happen in the near future, and you won’t be able to take advantage of the various benefits of DSDS without iOS 12.1 installed.