Keyed

#key names a child of a composite element. Its value becomes the array key of the row.

$form['options'] = [
  '#type' => 'element_multiple',
  '#title' => $this->t('Options'),
  '#key' => 'value',
  '#header' => TRUE,
  '#label' => $this->t('option'),
  '#labels' => $this->t('options'),
  '#element' => [
    'value' => [
      '#type' => 'textfield',
      '#title' => $this->t('Value'),
      '#title_display' => 'invisible',
      '#placeholder' => $this->t('Enter value'),
    ],
    'text' => [
      '#type' => 'textfield',
      '#title' => $this->t('Text'),
      '#title_display' => 'invisible',
      '#placeholder' => $this->t('Enter text'),
    ],
    'score' => [
      '#type' => 'number',
      '#title' => $this->t('Score'),
      '#title_display' => 'invisible',
      '#placeholder' => $this->t('Enter score'),
    ],
  ],
  '#default_value' => [
    'one' => ['text' => 'One', 'score' => 1],
    'two' => ['text' => 'Two', 'score' => 2],
  ],
];

A table with the columns Value, Text and Score and two rows.

The value is keyed by the first column. The key is not repeated inside the row:

[
  'one' => ['text' => 'One', 'score' => 1],
  'two' => ['text' => 'Two', 'score' => 2],
]

Disabled key

Set #disabled on the key child to lock the keys of the rows from #default_value. A browser does not send disabled fields, so the module restores the key on the server. The key stays with its row, also after the rows are reordered.

$form['codes'] = [
  '#type' => 'element_multiple',
  '#title' => $this->t('Codes'),
  '#key' => 'code',
  '#header' => TRUE,
  '#element' => [
    'code' => [
      '#type' => 'textfield',
      '#title' => $this->t('Code'),
      '#title_display' => 'invisible',
      '#disabled' => TRUE,
    ],
    'label' => [
      '#type' => 'textfield',
      '#title' => $this->t('Label'),
      '#title_display' => 'invisible',
    ],
  ],
  '#default_value' => [
    12 => ['label' => 'Twelve'],
    7 => ['label' => 'Seven'],
    30 => ['label' => 'Thirty'],
  ],
];

A table with the columns code and label and three rows and a blank row. The code column is disabled and its values 12, 7 and 30 cannot be edited.

Rules

Keys must be unique. A duplicate shows the error The Value 'one' is already in use. It must be unique.
A row with an empty key and empty content is dropped.
#key needs a composite #element. With a single element the module throws an \InvalidArgumentException.
Related property: #key.