How is Expression Transformation different from the Function Transformation?

In the Expression transformation, you can define an expression either from the library of built-in functions in the expression builder or write your own expression. You can also modify and use variables for various calculations in the expression transformation. Whereas in the Function transformation, you can only work with a set of built-in functions. You cannot modify or customize these built-in functions to suit your requirement, as you can do in the Expression transformation.