Remove required field in validation form If I have a multiple field (ajax) required: FIELD_A (N), and I want it not required if: FIELD_B has a certain value. How can I do ? Any function in $ form_state?
Use states to style the input as required. This won't actually perform any validation but will add the required asterisk to the field when field B has a certain value.

$form['FIELD_A']['#states'] = [
  'required' =&gt; array(
    ':input[name="FIELD_B"]' =&gt; array('value' =&gt; 'foo'),
  ),
];


Then use custom validation to actually check FIELD_A has the correct value depending on the value of FIELD_B.