Placing this template on a dialog page indicates — when viewing the dialog page via dialog action do verb view — that any internal notes about the current page view should be placed on the page at the position of each call to this template. Internal dialog notes are generated as low-profile diagnostic information explaining complex rejection-decisions that don't warrant an obtrusive error message, such as why a page does not provide outgoing authentication, or why it doesn't qualify for continuation of an action-sequence. Under ordinary circumstances the user doesn't want to know these things, but when debugging a dialog it can be extremely useful to have an explanation of why the software isn't doing something.
The view verb of action do processes calls to this template after all other processing has been completed; so the notes are not available as template parameters or even as dialog field values. This timing is warranted because some — probably most — notes are generated after both template parameters and dialog field values have been processed.