php - How does drupal choose which function to process a form submit? -

I'm really confused, can someone explain it?

Check entries for 'valid form' and 'form submission' - by default Doppel For a function that is named [form_id] _submit , but this is only the default behavior of the

form declaration (see) callback function in the #submit attribute By typing a name you can override it (or submit multiple callbacks).