What is the eventCategory for “Build result changed” event?

I found it always hard to find anything about event handlers. I don't have an example and I don't have a list of event categories. I am not aware of any examples. However, the basic extension mechanism works similar to a follow up action and the interfaces you must implement can be found in the extension.

For build I found BuildResultChangeEvent if following it up the extension hierarchy I find com.ibm.team.build.internal.common.events.BuildChangeEvent and there:

    public static final String BUILD_RESULT_CHANGED_EVENT_CATEGORY = "com.ibm.team.build.category.BuildResultChanged"; //$NON-NLS-1$

com.ibm.team.build.category.BuildResultChanged would be what I would try.

Yes, Ralph, you are right, thanks for your way to find the event category id.
I use the following extension to create a Build Result Change event followup actions:
       <extension point="com.ibm.team.process.service.eventHandlers">
        <eventHandler
            class="com.ibm.team.build.internal.service.process.event.tests.BuildProcessEventHandler"
            id="com.ibm.team.build.test.BuildProcessEventHandler"
            eventCategory="com.ibm.team.build.category.BuildResultChanged"
            name="Build Event Handler">
        <extensionService
            componentId="com.ibm.team.build"
            implementationClass="com.ibm.team.build.internal.service.process.event.tests.BuildProcessEventHandler">
          <prerequisites>
              <requiredService interface="com.ibm.team.repository.service.IRepositoryItemService"/>
              <requiredService interface="com.ibm.team.build.internal.common.ITeamBuildService"/>
          </prerequisites>
        </extensionService>
      </eventHandler>
   </extension>
Here is a example code from BuildProcessEventHandler class.

public class BuildProcessEventHandler extends AbstractService implements IChangeEventHandler {

    /*
     * (non-Javadoc) Intentionally not documented. See parent.
     */
    public void handleEvent(IChangeEvent event, IProcessConfigurationElement handlerConfiguration)
            throws TeamRepositoryException {

        if (BuildResultChangeEvent.BUILD_RESULT_CHANGED_EVENT_CATEGORY.equals(event.getCategory())) {
            IRepositoryItemService repoService = getService(IRepositoryItemService.class);
            IBuildResult buildResult = (IBuildResult) repoService.fetchItem(event.getItem(),
                    IBuildResult.PROPERTIES_COMPLETE);
            buildResult = (IBuildResult) buildResult.getWorkingCopy();
            buildResult.setTags(getClass().getSimpleName());
            ITeamBuildService teamBuildService = getService(ITeamBuildService.class);
            teamBuildService.save(buildResult);
        } else {
            throw new IllegalArgumentException("Why am I being invoked for a non-build event?"); //$NON-NLS-1$
        }
    }
}