VirtualBox comes with comprehensive support for third-party developers. The so-called "Main API" of VirtualBox exposes the entire feature set of the virtualization engine. It is completely documented and available to anyone who wishes to control VirtualBox programmatically.
The Main API is made available to C++ clients through COM (on Windows hosts) or XPCOM (on other hosts). Bridges also exist for SOAP, Java and Python.
All programming information (documentation, reference information, header and other interface files as well as samples) have been split out to a separate Software Development Kit (SDK), which is available for download from http://www.virtualbox.org. In particular, the SDK comes with a "Programming Guide and Reference" in PDF format, which contains, among other things, the information that was previously in this chapter of the User Manual.