In order to construct and update the profile of the active user ua (user for
which recommendations must be provided) her reactions to items are collected in
some way and recorded in the repository Feedback. These reactions, called annotations [39] or feedback, together with the related item descriptions, are exploited
during the process of learning a model useful to predict the actual relevance of newly
presented items. Users can also explicitly define their areas of interest as an initial
profile without providing any feedback.