Sitecore Source Property
The Sitecore source property is used in an endless number of contexts within Sitecore. In this example, it’s a property on the Multilist field “UpComingNews”. It’s one of my favourite “clever” bits of Sitecore in that it can take an amazing number of formats. You can choose a node in the Content Tree, or use a complex XPath or search query, or in this case, it actually references a custom class.
What it enables is extremely complex (and often dynamic) relationship options between Sitecore items when authors are creating content. While Sitecore has a Content Tree which is commonly used as the primary means of establishing a site structure, there are actually two other levels; item references (such as shown in the example) and also component references when you are building a page layout.
This simple bit of boring and often overlooked functionality is one of the more powerful conceptual tools in Sitecore.
(As a bit of commentary, Sitecore has a lot of these areas with amazingly powerful functionality which are not given proper UX prominence and often poorly documented - Tony Byrne calls this “secret knowledge” within the Sitecore community a “guild culture” and notes that Drupal is similar in this respect).