Magento 2: Understanding the links default in uiElement Objects

When Magento 2 creates a new constructor functions based on the uiElements constructor functions, Magento 2 will often use a links default

return Element.extend({
    defaults: {
        /* ... */
        links: {
            value: '${ $.provider }:${ $.dataScope }'
        }
        /* ... */            
    },
/* ... */        

The links property looks similar to the imports and exports feature, in that it uses template literals to create an identifier string.

{
    value: 'checkoutProvider:shippingAddress.firstname'
}

When you (or Magento) use a links value, you’re both importing and exporting the properties from the specified registry entry. If you’re not familiar with importing and exporting, you can read about them in the Magento 2: uiClass Data Features article, part of the UI Components series. The above value translates to the following uiRegistry key

reg = requirejs('uiRegistry');
reg.get('checkoutProvider').shippingAddress.firstname;

For the curious, these links are setup in the following method

#File: vendor/magento/module-ui/view/base/web/js/lib/core/element/element.js
initLinks: function () {
    return this.setListeners(this.listens)
               .setLinks(this.links, 'imports')
               .setLinks(this.links, 'exports')
               .setLinks(this.exports, 'exports')
               .setLinks(this.imports, 'imports');
},