This level contains all messages that a component or a module can generate; for many communication modules the generated messages include all data sent and received via a communication link.

This level contains messages with low-level information, as transfer protocol messages and commands, etc.

Messages of this level are about temporary problems (for example, busy fax numbers)

Messages of this level are about successful message transfer.

Messages of this level are about unrecoverable delivery failures only; they do not include messages about temporary problems. 

Messages of this level appear only if a really bad problem has occured (like a file system crash)