The "vu" criteria is applied to a document<->object association (since a single given document can be linked to several objects: articles, sections, etc.). It is also updated upon any modification of such an object.
If the document is "integrated" into an object (typically using <imgXX> or <docYY|left> or even with <embZZ> in the text of an article for example), then such a document-object association will be classified as "vu = oui", i.e. the document in question has been "seen" already by the object.
For example, when displaying the portfolio of documents for an article, we can include within an articles loop another documents loop that applies the criteria {vu = non}: this will have the affect of only displaying in the portfolio those documents that have not already been displayed within the body of that same article.