From SMS Wiki
Tariff is used when sending premium SMS messages. This is a floating point value in currency as per the country of your Short Code. For example, if you have a US short code and set tariff = 0.50, the user will be billed US$ 0.50 by his/her service provider.
- You must have a premium enabled Short Code assigned to your account to use this option
- You can bill on MT only
- You cannot bill on Mandatory Keywords. For example, when someone sends message with content HELP, you must reply with standard help text as per your Short Code application. This message is free (standard carrier charges apply)