All tests are grouped in suites. All main suites are defined in pom.xml file in maven-surefire-plugin plugin configuration:

<suiteXmlFiles>
    <suiteXmlFile>src/test/resources/testng_server.xml</suiteXmlFile>
    <suiteXmlFile>src/test/resources/testng_muc.xml</suiteXmlFile>
    <suiteXmlFile>src/test/resources/testng_pubsub.xml</suiteXmlFile>
    <suiteXmlFile>src/test/resources/testng_http_api.xml</suiteXmlFile>
    <suiteXmlFile>src/test/resources/testng_archive.xml</suiteXmlFile>
    <suiteXmlFile>src/test/resources/testng_custom.xml</suiteXmlFile>
    <suiteXmlFile>src/test/resources/testng_jaxmpp.xml</suiteXmlFile>
</suiteXmlFiles>

For the actual semantics of the suite xml files please refer to Test-NG documentation.