I create a new entity and am asked for a title (obvious), and there is a http://schemaapp.com URI provided. What is the advantage of using this, instead of provided the URI to my website, or to Wikidata, or somewhere else. Are other third-party URIs available and valid?
When you create a new “thing”, in this example an Organization, we prompt you to name it, add a URI and define the type. The name should be a common name that helps you reference what this markup is about.
The URI most commonly is the webpage URL you are adding the markup to, or a reference to where that “thing” is defined (e.g. Wikipedia entry). The http://schemapp.com… is used if a URL is not specified so we can connect all the markup together in our graph database. It should only be left if there is not a specific webpage or definition page available.
If we were adding the organization markup to our homepage www.schemaapp.com, we would add https://www.schemaapp.com/ as our URI. This URI is also used by the WordPress plugin, Google Tag Manager or API to pull in the correct JSON-LD.
If you were adding a reference to a city or a language, you might make the URI the wikipedia page for that city or language.
See examples of when we do this in our Local Business Tutorial.