To follow up with what Lluis Salvador @l_salva posted.
The diag infrastructure is much different than the CCMS infrastrucutre.
To see all of your Diag agents you actually pull up a web page and can review online vs offline agents along which software versions of each agent.
That being said I can also tell you that after I set them up for my environment, I never review that web page unless there is an issue; I rely on Solution Manager to notify me if communications between itself and an agent is down or is degrading. If I recall for CCMS it was just a down alert if the sapccms RFC stopped working.