Does the $form['element']['#disabled'] = TRUE guarantee that the user cannot edit the field or is it just a UI setting I have a form element like this:

$form['title'] = array(
  '#type' =&gt; 'textfield', 
  '#title' =&gt; t('Subject'), 
  '#default_value' =&gt; $node-&gt;title, 
  '#size' =&gt; 60, 
  '#maxlength' =&gt; 128, 
  '#disabled' =&gt; TRUE,

);


Does this make the field 100% non-editable by the user or is it still possible to set a value for this field using manual form submission using jQuery / Postman to the action URL?
If you disable field in backend, the field value won't be actually changed even if you change it in HTML or by JS.
You can test it:
 - disable element in form alter
 - open element in Chrome inspector (or other) and remove disabled attribute
 - change value in field and save the form
 - changed value will not be saved