How is the `block` widget parameter type defined and used?

1. The definition

2. The usages

2.1. Magento\Catalog\Block\Product\Widget\Link («Catalog Product Link»)

2.2. Magento\Catalog\Block\Category\Widget\Link («Catalog Category Link»)

2.3. Magento\Cms\Block\Widget\Page\Link («CMS Page Link»)

2.4. Magento\Cms\Block\Widget\Block («CMS Static Block»)