Here is a list of all documented namespace variables with links to the namespaces they belong to:

$allLabels : Phpml\Helper
$buffersize : tool_log\helper
$component : tool_log\helper
$count : tool_log\helper
$jsonformat : tool_log\helper
$matrixhomeserverurl : communication_matrix
$quiz : mod_quiz\tests
$store : tool_log\helper
$user : mod_quiz\tests
ANTIVIRUS_CLAMAV_SOCKET_CHUNKSIZE : antivirus_clamav
ANTIVIRUS_CLAMAV_SOCKET_TIMEOUT : antivirus_clamav
buffered_writer : tool_log\helper
H5PExport : Moodle
logging_trait : core\task
question_helper_test_trait : mod_quiz\tests
store : tool_log\helper