Each Form Field has a possibility to dynamically set its state: visibility, possibility to edit or only to show, and also dynamic validation conditions. This can be set on Field configuration window, but requires firstly to set form as 'Asynchronous Form':

When this is checked on each field you can find Display&Validation Conditions sections:

This section contains 3 subsections, where you can configure dynamic conditions that defines 3 below areas of field functionality:

• Visibility Condition
  If condition is true field is visible, otherwise hidden.
• Editability Condition
  If condition is true field can be edited, otherwise is not editable.
• Validation Condition
  If condition is false Validation Message is shown and form cannot be submitted.

Note: All of conditions can be advanced set of and and or conditions.

Note: If you want to base condition on other fields values please use lookup to Workflow Data -> Current Form -> Form Fields.

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