They can either be inline with a NOTE tag and included with the information they pertain to, or they can be a link to a NOTE record. The NOTE record allows the same note to be used multiple times.
The Note Details section only shows NOTE records that are used two or more times, or NOTE records that are unused and thus unreferenced. NOTE records that are only used once are not included in the Note Details section, but are shown inline where they were linked from.
The Note Details section is only shown if there is at least one NOTE record used 2 or more times or at least one unused NOTE record. Not all genealogy software exports its notes as NOTE records, so often this section does not appear.
This is how a note link to a NOTE record is displayed. Only the first line of the note is displayed:
References to each record or event that links to that NOTE record are given. They include the name of the person (as a hyperlink to the person), the number of the person, and if it is an event, then also the date and the place of the event.