Apache Axis2/C contains both applications, like HTTPD, and libraries, like APR. It also contains a number of related sub-projects. It allows third-party service modules to be written against its API (such as HTTPD), and may also compile its own third-party modules (e.g., mod_axis2) to be used with HTTPD.
These complexities probably mean that we need to develop a fairly comprehensive set of versioning rules.
Being an Apache project, the Axis2/C project follows the APR guidelines on versioning. http://apr.apache.org/versioning.html