Critical - ?
Trouble - A threshold or limit has been exceeded which could indicate an issue. Information purposes only at this stage unless a manual review deems it appropriate to action [This is my definition].
Attention - ?
Service Down – The service being monitored is offline and cannot be accessed. This would require manual intervention to resolve (restarting a service etc). [This is my definition]
Clear - A previously reported issue has been resolved. This can be automatic or via a manual clear flag placed on the issue. [This is my definition]