Currently you can only tag instance level objects, being able to tag clusters and nodes would greatly help us with our downtime automation, among other things. Not going to go into great detail, but being able to tag all objects in the tool the same way you can with the instances would have some far reaching uses for a lot of people I would think.

Related support request with details:

https://productsupport.red-gate.com/hc/en-us/requests/365402