Composite element

A composite element repeats a group of inputs. Pass an associative array of children in #element. Each key is the name of a value.

$form['contacts'] = [
  '#type' => 'element_multiple',
  '#title' => $this->t('Contacts'),
  '#element' => [
    'first_name' => [
      '#type' => 'textfield',
      '#title' => $this->t('First name'),
    ],
    'last_name' => [
      '#type' => 'textfield',
      '#title' => $this->t('Last name'),
    ],
  ],
  '#default_value' => [
    ['first_name' => 'John', 'last_name' => 'Smith'],
    ['first_name' => 'Jane', 'last_name' => 'Doe'],
  ],
];

Two rows with a first name and a last name field stacked in one cell, each with its label.

The value is a list of arrays:

[
  ['first_name' => 'John', 'last_name' => 'Smith'],
  ['first_name' => 'Jane', 'last_name' => 'Doe'],
]

Notes

Without #header, the children are stacked in one cell and keep their labels. To get one column per child, use #header. See Composite with header.
A row is dropped when every child is empty.
A child can be any form element, also a container. See Nested containers.
Related properties: #element, #default_value.