The Formula property is available for Calculated fields. It is used to define the Formula to automate a calculation in your Form.
Take a look at all the available Formulas in the Formulas Manual.
To make the Formula more readable and easy to use, make sure to add Codes to the fields you want to use.
When the Formula is valid, the field will be marked in green.
You can also write a Formula using the Formula editor.
Formulas can also be used to define advanced validation and relevance rules when designing a Form.