I just started reading the SharePoint Products and Technologies Protocol Documentation today, quite interesting and profound to found out the level of details released under this document. All the specification of protocols (sometimes I would like to call it APIs) used by SharePoint from system behaviors, WSDL, XML Schemas, managed (and/or unmanaged) APIs as well as Stored Procedures used by SharePoint. Technologies from CAML, SharePoint list, view, search, workflow, administration all the way to the user interface is pretty well covered in this set of documents.
For Microsoft partners it opens up a whole new opportunities for them to build applications on top of SharePoint. (Almost) gone are the time when we try to figure out why SharePoint behave a certain way and there are no official documents to look at. Most of the time we resort to word by word advices from fellows developers in the forum. Imagine now it is possible for developers to write application (esp. those with different client platform) to works seamlessly with Microsoft SharePoint platform.
Besides SharePoint technologies, Microsoft also release Protocol documentations for Windows(incl. .NET Framework), Office and Exchange and also XAML (the markup language for Windows Presentation Foundation and Silverlight); all of these fall under the so called High Volume Products from Microsoft.