As a general rule you can only access the value of a Field on the Form you have open. If you are building a Multi-form project, then you are likely to want to access the value of Fields on other Forms within your project. You can do this a couple of ways.
Using ‘cloned’ Fields on each Form
The first, and most straight-forward, is to create a new simple ‘text field’ on your Form and give it the same name as the Field on the other Form. The second Field becomes a ‘clone’ of the original Field on the other Form.
When the two Forms load as a Multi-Form, the value of the Field will be loaded into both Forms, so if you change the value on one Form, it will change the value on the other Form. Furthermore, you will be able to access the value of the Field on both Forms.
If you only want users to be able to change the value on one of the Forms, then you make the Field a ‘hidden‘ Field on the other Form.
This method will enable you to use the Field value in the Rules Wizard and in Scripts.
The second method is to use mydata in the form script, a topic covered here