The following states of terminals are possible:

OK: terminals without errors or warning that have successfully sent the Heart-Beat to the server.
Not OK: terminals that have some errors or/and warnings or those that have not sent their Heart-Beat in a due time.
Failed: terminals that have not sent their Heart-Beat in a due time.
Error: terminals with some errors (e.g. the memory state is critical).
Warning: terminals with some warnings that might be helpful to prevent errors.