Validation will allow you to define valid values for a field. If the user enters an incorrect value, then an error will be displayed to encourage the user to change it.

If the Validation script returns true, the attribute value is valid , if it returns false then the value is not valid and an error message is returned.

Adding a custom validation error message

<pre class="wp-block-syntaxhighlighter-code">// Validate that a Field attribute value is between 10 and 20.
//  - If the value is not valid return a custom error message.  
if( f < 10 || f > 20 ) {
  // value is valid
  return true
} else {
  // value is not valid, so return a custom error message
  return "Value " + f + " is not allowed"
}