The value of a custom variable can be a literal or an expression:

If you want the variable to contain a literal value, set the value of its Value property.

If you want the variable to contain an expression, so that you can use the results of the expression as its value, set the EvaluateAsExpression property of the variable to true, and provide an expression in the Expression property. At run time, the expression is evaluated, and the result.

For more details LINK