How do SMS messages fit into flows with emails and non-SMS subscribers?

You can use an SMS message anywhere in a flow that you would use an email action. You can have SMS and email messages both within the same flow. A use case for this would be, for example, if you wanted to send a text that says "check your email for a surprise" and then send an email that contains a surprise. Make sure to split out customers who have not opted into SMS. Recipients who are not specifically opted in to SMS will be skipped in from receiving the SMS message, with the skipped reason Missing SMS Consent.