In your SMS message you can present an expression whose value is computed at runtime and can use data from the eSpace. This behavior is implemented by the SMS Expression widget.
An expression is a combination of values and operators, or it can be the result of a function. In Service Studio you have a specific editor to handle expressions; however, you can just type the expression in its Properties window.
The maximum length of an expression is 50 Kb; Service Studio performs this evaluation at development time.
SMS Expression Widget Properties