From Joomla! Documentation
The "API17" namespace is an archived namespace. This page contains information for a Joomla! version which is no longer supported. It exists only as a historical reference, it will not be improved and its content may be incomplete and/or contain broken links.
Classes with related functions within the Joomla Framework or Platform are gathered together into groups called subpackages. Each subpackage has its own directory within the /libraries/joomla directory. For example, the Application subpackage consists of all the classes within the /libraries/joomla/application directory.
The following subpackages are available in this version:-