Activation env field vs trackId env field


When I publish canonical from ATC agent (webmethods4.1.1) neither blueprint and workunit set the field _env.activation.
On my project, helpdesk has developed a web monitoring tool wich display as transactionId the field _env.activation.
Does someone know the diofference between activation and trackId ? Why activation is empty when I publich event from ATC and there is one when event is sent from IntegrationComponent ?
Do I have to set an activation on all blueprints and workunits with java code ? Or is there an other way to set it ?