Salesforce Identity Services: A Mobile-First World Requires Mobile-First...

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It's a mobile-first world. Today, most people prefer to engage with apps through mobile devices. And a growing number of people use mobile devices exclusively. So, the first step to increasing customer engagement is to make it easy for your customers to sign up and log in on the go. You can let external users log in with something they won't forget—like their email address or phone number. A simpler, passwordless login process encourages more customers to engage. You can set up this on-trend login process in a few clicks. This feature is available to all communities for external users.
Quicker and Easier Logins: https://developer.salesforce.com/docs/atlas.en-us.216.0.externalidentityImplGuide.meta/externalidentityImplGuide/external_identity_passwordless_login_about.htm
railhead: Identity for Customers
https://trailhead.salesforce.com/en/content/learn/modules/identity_external