An event in a user's history. Events are associated with items. In some
cases, it makes sense that a user may have events not associated with an item
(such as login/logout events). We recommend that integrators use a designated
item ID in such cases. If that item type never shows up in events used by
recommenders, then it should not affect the recommenders.
Events are immutable. Deviations must be clearly documented and only
used in very limited cases (e.g. to implement fast iterators).