Locale Field Type
When creating Data Extensions for SMS sending in SFMC, Salesforce requires a locale data type to ensure that SMS messages have the correct country prefix when being sent. To this end, you must include a locale field, using either Fixed or Dynamic Custom Values depending on your case.
Single locale
If your data belongs to one locale, you can use Fixed Custom Value to specify a default locale (e.g., 'en-US').
Multiple locales
If your data belongs to multiple locales, you can use Dynamic Custom Value to specify the locale for a specific area (e.g., if the country = 'United States', then value = 'en-US', if the country = 'Canada', then value = 'en-CA').