You can add a Rule to a Form by going to the Main Form Configuration (clicking on the Cogwheel icon in any record).
From the Advanced drop down menu select Form Rules.
Click the “+” icon to insert a new Form Rule.
The Form Rule window will popup where you can add a description, enable or disable your form rule, select the form this form rule will apply to (Main, Supplier or Both), and define conditions, triggers and actions.
Note: Only an action is required to create a form rule. Not having a condition will always apply the action, and not having a trigger will apply the first action on save of the form.
Defining a condition will let the application know when/if to perform the action.
Defining a trigger field will let the application know that it can perform an action once that trigger field is modified.
Defining an action will let the application know what to perform.