Messages to clients are sent via SMS or email. The fallback functionality helps ensure messages are delivered when client’s profile is missing an email or cellphone number or you have run out of SMS tokens.
If you're using, as an example, e-mail with SMS as fallback:
When an email address is not available, but a cellphone number is available, an SMS will be sent
When an email address is available, regardless if a cellphone number is available or not, an email will be sent
And vice versa for using SMS with email as fallback.
Read this article to help you set up your automated messages.
When using SMSs with email as fallback, emails will be sent if you run out of SMS tokens.
Note: Fallback messages will not be sent if delivery fails, for whatever reason. For example, if an email address is incorrect or phone number does not exist.