Messages in the Dynamic Expression Editor 
When you click Check in the Dynamic Expression Editor dialog box, the validation results are displayed in the area below the Expression pane. Validation results can include the following:
If an expression is correct, it is confirmed with a green flag and the message Expression is valid is displayed.
If an expression uses invalid syntax, an error (yellow flag) or warning (red flag) is displayed, together with information that helps you to correct the syntax.
When there are data type mismatches, the Dynamic Expression Editor implicitly converts types where possible. For example, when it expects data of type String, but the validation result is Boolean, the editor confirms the expression and generates a warning that suggests that you use a different function (or else accept the implicit conversion). You can choose to proceed with the defined expression or you can modify it.
If the type conversion is problematic – such as attempting to convert data of type Date to type Boolean – the editor generates an error message. You can choose to proceed with the defined expression, or you can modify it.