In Settings > Users > Registration, there are options for registration by email and mobile. Checking or unchecking these options or a combination thereof will provide different registration options for your users.
No options selected
By default, an email will be used for registration anyway, even if no options are checked and email is technically 'not required'. As either an email address or mobile number is needed by the Good Grants system to create the user profile, users will still have to provide, at minimum, an email address to register and must also confirm their registration via an email. No field will be available to enter a mobile number.
Only email selected
Users must register via email and will receive an email to confirm their registration. This setting is essentially the same as leaving all options unchecked.
Email required, mobile registration enabled
With the first two options checked, users must enter an email address and confirm their registration via an email. The field for entering a mobile number will be available but optional. Users cannot choose between the two options with regards to the confirmation step; they will be required to confirm via email.
Only mobile registrations checked
With only the mobile option checked, users can choose to register via email or mobile. If they register via email, they will be sent a confirmation link via email. If they register via mobile without entering an email address, they will be sent a confirmation link via SMS.
Mobile registration with Mobile required checked
If the mobile option is checked and made required, users can still register via email but must enter a mobile number. If they enter an email address, they will be sent a confirmation link via email. If they only enter the mobile number as required, they will receive the confirmation link via SMS.
All three options checked
Users must enter an email address and must enter a mobile number but the confirmation link will be sent via email. There is no option to choose how they receive the confirmation; email will always be the default.
In conclusion, the only way users will receive an SMS instead of an email to confirm their registration is:
1) if only mobile registration is checked OR 2) if only mobile registration plus mobile required is checked AND (for both scenarios) the user chooses not to enter an email address.
As soon as an email address is included, the confirmation will be sent via email, irrespective of whether a mobile number has been entered.