The Formula step can calculate Formula Expressions within a data stream. It can be used to create simple calculations like [A]+[B] or more complex business logic with a lot of nested if / then logic.
When you press on the Formula cell entry, the formula editor window opens and gives you help on the available functions:
The formula editor window does a direct check on the syntax and the field names supplied. In case of an error you get the error message direct, for instance: