Within your plugin.json you can define the type of plugin you are creating. This will be used for GoPage to know how to load the plugin, and when to allow the user to select it.
The item type Plugin will be available when creating a new Link Item, allowing it to be the center, right, or left plugin for said Item. Once installed this will load the main file in your package.json as a new script for the page, and as such should point to a JavaScript file.
The theme type Plugin will not show any configuration options to the user and once installed will be applied globally to GoPage and will target your main file as a stylesheet that overrides the current theme of the system.
The header type Plugin will be shown only in the Header of GoPage and shouldn't expect to have any data-url attributes. It can still of course have data-config attributes. And is intended to show data that doesn't apply to any links but rather show data that applies to the user, such as temperature, or the Air Quality Index like AQICurrent.