What is attr('data-user-id') doing?

Here's the code that's getting triggered by the "user-form-show" method:

$(window).on('user-form-show', function(e, userId, opts){
    // prep the modal
    $uModal.find('.loading').show();
    $uModal.find('.main').hide();

    // show the modal loading screen
    $.magnificPopup.open({
      alignTop: true,
      items: {
        src: $uModal, // can be a HTML string, jQuery object, or CSS selector
        type: 'inline'
      }
    });

    // clear the cached data
    data = {};

    // clear the form from any previous data
    clearForm();
           

    // load the data if a valid id is provided
    if (typeof userId != 'undefined' && userId != 'new') {
      loadFormData(userId);
    } else {
      // set the id for a new user
        loadedUserId = 'new';
        // load drag & drop members 
        teamDragula(userId);

        $form.find('#user-role').attr('data-user-id', loadedUserId);
      // force the password fields
      $form.find('.btn-change-password').replaceWith( $passwordForm.clone() );
      $form.find('.btn-cancel-password').closest('.col').remove();

      // show the form
      $uModal.find('.loading').hide();
      $uModal.find('.main').show();
    }
  });

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The line that I'm wanting to understand is this:

$form.find('#user-role').attr('data-user-id', loadedUserId);

Earlier in the file, $uModal was defined like this:

var $uModal = $('#modal-user-form'),
      $form = $('#user-form'),
      $passwordButton = $form.find('.btn-change-password').clone(),
      $passwordForm = $form.find('.password-template').clone().removeClass('d-none'),
      data = [],
      loadedUserId = '',
      passwordRegex = {
        upper: /[A-Z]{1,}/,
        lower: /[a-z]{1,}/,
        number: /[0-9]{1,}/,
        special: /[\!\@\#\$\%\^\&\*]{1,}/,
        total: /.{8,}/
      };

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When you follow the trail as far as the "user-form.html.twig," you find that the portion of code that I've got in bold is referring to this:

<select id="user-role" name="user[role]" class="form-control mdb-select md-form" required>
               {% if roles is not empty %}
                {% for role in roles %}                
                    <option value="{{role}}" {{role=='Sales Representative'?'selected="selected"':''}} >{{role}}</option>
                {% endfor %}
                {% endif %}
            </select>

My question is: Where is "data-user-id?"

I'm thinking that the referenced SELECT would have a "user-id" value attached to is and the ".attr('data-user-id', loadedUserId" code is populating that value with the "loadedUserId" value, yes?

What is "att('data-user-id', loaded.UserId)" doing?