https://wccftech.com/ios-12-for-iphone-x-lets-you-set-up-an-alternate-appearance-for-face-id/

Don’t confuse it with ‘multiple users.’ iOS 12 for iPhone X has a new ‘Set Up an Alternate Appearance’ option in Face ID settings. Here’s what it does.

Unlike Touch ID, Face ID lets you just input just one print and that’s all you get. But once you do scan your face, your iPhone X will learn over time what you look like and if whether or not your face is changing as well. Of course, some changes happen a lot more drastically, if you happen to use makeup or wear a certain set of spectacles and whatnot. In that case, Face ID might just end up failing over and over again until or unless you come out of your ‘alternate’ look.