Messages
Feedback, status and help messages are used to communicate with users after a task: to confirm that a task has been done, to tell that the platform is processing the task and needs a bit more time, to ask users to do the task one more time as the request couldn't be performed, to show users what actions can be done and where, or to indicate the state of a document. A feedback message is always a good way to show users what happened or what is happening: the content of the message is as important than the presence of the message itself. The fewer surprise there is during a process, the more intuitive the interface will be.
If you want to display a feedback message under a field in a form, use only the class of the message type.