Technically, WordPress determines if there is an upgrade by comparing the available version at WordPress.org to the version installed. This is done through a request (hypertext transfer protocol, or HTTP) to WordPress.org and comparing version information in the database.
Similarly, plugins have a nag that comes in the form of a small red bubble that is superimposed on the Plugins navigation item (see Figure 15.2). The number in this bubble represents the number of plugins that are installed (active and inactive) that have upgrades available. This is based on information from the WordPress plugin repository, another valuable reason to include General Public
License (GPL)-compatible plugins in the repository. If the plugin is listed in a different repository or self hosted (via a blog, wiki, or other Web site), bloggers will not have the benefit of upgrade notifications.
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