Both system and user attributes (except for read-only ones) can be given a list of suggested values. This is a means of pre-populating attribute value pop-up lists with values that may not yet be in use by any note. Suggested values apply to String/Set/List data types only.
Suggested values always appear in the value menu of Displayed Attributes, and are always offered for autocompletion, even if no notes currently use them. Suggested values may be added to attributes via the 'Suggested' box on the system and user tabs of the Document Inspector.
Suggested values do not show up in values(), collect() and such, if not actually used by any note. Pop-up value lists do not indicate which are suggested values. Thus it is not possible to tell apart currently used values solely by inspecting a pop-up that includes suggested values.