Type – Checkboxes

'font_subset' => [
  'label' => __( 'Google Fonts Subset' ),
  'type' => 'ct-checkboxes',
  'attr' => [ 'data-columns' => '2' ],
  'allow_empty' => true,
  'choices' => [
    'latin-ext' => __( 'Latin Ext' ),
    'vietnamese' => __( 'Vietnamese' ),
    'cyrillic' => __( 'Cyrillic' ),
    'cyrillic-ext' => __( 'Cyrillic Extended' ),
    'greek' => __( 'Greek' ),
    'greek-ext' => __( 'Greek Extended' ),
  ],
]

Value Format

'font_subset' => [
  'latin-ext' => true,
  'vietnamese' => false,
  'cyrillic' => false,
  'cyrillic-ext' => false,
  'greek' => false,
  'greek-ext' => false,
],